Each recipe consists of 3 main parts: defining identifiers, setting build
variables, and defining build commands.

The package "mylib" will be used here as an example

General tips:
- mylib_foo is written as $(package)_foo in order to make recipes more similar.
- Secondary dependency packages relative to the bitcoin binaries/libraries (i.e.
  those not in `ALLOWED_LIBRARIES` in `contrib/devtools/symbol-check.py`) don't
  need to be shared and should be built statically whenever possible. See
  [below](#secondary-dependencies) for more details.

## Identifiers
If package does not define a `$(package)_local_dir` variable, it is required to
define these variables:

    $(package)_version:
    Version of the upstream library or program. If there is no version, a
    placeholder such as 1.0 can be used.

    $(package)_download_path:
    Location of the upstream source, without the file-name. Usually http, https
    or ftp. Secure transmission options like https should be preferred if
    available.

    $(package)_file_name:
    The upstream source filename available at the download path.

    $(package)_sha256_hash:
    The sha256 hash of the upstream file

If a package does define a `$(package)_local_dir` variable, the above variables
are not required and will be ignored.

These variables are optional:

    $(package)_build_subdir:
    cd to this dir before running configure/build/stage commands.

    $(package)_download_file:
    The file-name of the upstream source if it differs from how it should be
    stored locally. This can be used to avoid storing file-names with strange
    characters.

    $(package)_dependencies:
    Names of any other packages that this one depends on.

    $(package)_patches:
    Filenames of any patches needed to build the package

    $(package)_extra_sources:
    Any extra files that will be fetched via $(package)_fetch_cmds. These are
    specified so that they can be fetched and verified via 'make download'.

## Local packages

If a package defines a `$(package)_local_dir` variable, the specified directory
will be treated as a download source, and a tarball of its contents will be
saved to `sources/`. A hash of the tarball will also become part of the package
build id, so if the directory contents change, the package and everything
depending on it will be rebuilt. For efficiency, the tarball is cached once it
has been created, but if the local directory is touched, it will be rebuilt.

Local packages can be useful for using git submodules or subtrees to manage
package sources, or for testing local changes that are not available to
download from an external source.

## Build Variables:
After defining the main identifiers, build variables may be added or customized
before running the build commands. They should be added to a function called
$(package)_set_vars. For example:

    define $(package)_set_vars
    ...
    endef

Most variables can be prefixed with the host, architecture, or both, to make
the modifications specific to that case. For example:

    Universal:     $(package)_cc=gcc
    Linux only:    $(package)_linux_cc=gcc
    x86_64 only:       $(package)_x86_64_cc = gcc
    x86_64 linux only: $(package)_x86_64_linux_cc = gcc

These variables may be set to override or append their default values.

    $(package)_cc
    $(package)_cxx
    $(package)_objc
    $(package)_objcxx
    $(package)_ar
    $(package)_ranlib
    $(package)_nm
    $(package)_cflags
    $(package)_cxxflags
    $(package)_ldflags
    $(package)_cppflags
    $(package)_config_env
    $(package)_build_env
    $(package)_stage_env
    $(package)_build_opts
    $(package)_config_opts

The *_env variables are used to add environment variables to the respective
commands.

Many variables respect a debug/release suffix as well, in order to use them for
only the appropriate build config. For example:

    $(package)_cflags_release = -O3
    $(package)_cflags_i686_debug = -g
    $(package)_config_opts_release = --disable-debug

These will be used in addition to the options that do not specify
debug/release. All builds are considered to be release unless DEBUG=1 is set by
the user. Other variables may be defined as needed.

## Build commands:

  For each build, a unique build dir and staging dir are created. For example,
  `work/build/mylib/1.0-1adac830f6e` and `work/staging/mylib/1.0-1adac830f6e`.

  The following build commands are available for each recipe:

    $(package)_fetch_cmds:
    Runs from: build dir
    Fetch the source file. If undefined, it will be fetched and verified
    against its hash.

    $(package)_extract_cmds:
    Runs from: build dir
    Verify the source file against its hash and extract it. If undefined, the
    source is assumed to be a tarball.

    $(package)_preprocess_cmds:
    Runs from: build dir/$(package)_build_subdir
    Preprocess the source as necessary. If undefined, does nothing.

    $(package)_config_cmds:
    Runs from: build dir/$(package)_build_subdir
    Configure the source. If undefined, does nothing.

    $(package)_build_cmds:
    Runs from: build dir/$(package)_build_subdir
    Build the source. If undefined, does nothing.

    $(package)_stage_cmds:
    Runs from: build dir/$(package)_build_subdir
    Stage the build results. If undefined, does nothing.

  The following variables are available for each recipe:

    $(1)_staging_dir: package's destination sysroot path
    $(1)_staging_prefix_dir: prefix path inside of the package's staging dir
    $(1)_extract_dir: path to the package's extracted sources
    $(1)_build_dir: path where configure/build/stage commands will be run
    $(1)_patch_dir: path where the package's patches (if any) are found

Notes on build commands:

For packages built with autotools, $($(package)_autoconf) can be used in the
configure step to (usually) correctly configure automatically. Any
$($(package)_config_opts) will be appended.

Most autotools projects can be properly staged using:

    $(MAKE) DESTDIR=$($(package)_staging_dir) install

## Build outputs:

In general, the output of a depends package should not contain any libtool
archives or `.pc` (`pkg-config`) files. Instead, the package should output
`.cmake` (CMake) files where possible.

From the [Gentoo Wiki entry](https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Handling_Libtool_Archives):

>  Libtool pulls in all direct and indirect dependencies into the .la files it
>  creates. This leads to massive overlinking, which is toxic to the Gentoo
>  ecosystem, as it leads to a massive number of unnecessary rebuilds.

Where possible, packages are built with Position Independent Code. Either using
the Autotools `--with-pic` flag, or `CMAKE_POSITION_INDEPENDENT_CODE` with CMake.

## Secondary dependencies:

Secondary dependency packages relative to the bitcoin binaries/libraries (i.e.
those not in `ALLOWED_LIBRARIES` in `contrib/devtools/symbol-check.py`) don't
need to be shared and should be built statically whenever possible. This
improves general build reliability as illustrated by the following example:

When linking an executable against a shared library `libprimary` that has its
own shared dependency `libsecondary`, we may need to specify the path to
`libsecondary` on the link command using the `-rpath/-rpath-link` options, it is
not sufficient to just say `libprimary`.

For us, it's much easier to just link a static `libsecondary` into a shared
`libprimary`. Especially because in our case, we are linking against a dummy
`libprimary` anyway that we'll throw away. We don't care if the end-user has a
static or dynamic `libsecondary`, that's not our concern. With a static
`libsecondary`, when we need to link `libprimary` into our executable, there's no
dependency chain to worry about as `libprimary` has all the symbols.

## Build targets:

To build an individual package (useful for debugging), following build targets are available.

    make ${package}
    make ${package}_fetched
    make ${package}_extracted
    make ${package}_preprocessed
    make ${package}_configured
    make ${package}_built
    make ${package}_staged
    make ${package}_postprocessed
    make ${package}_cached
    make ${package}_cached_checksum