edde96376a cmake: Respect user-provided configuration-specific flags (Hennadii Stepanov)
Pull request description:
This PR addresses [this](https://github.com/bitcoin/bitcoin/issues/31491#issuecomment-2542140874) comment:
> I suppose that should only happen if the `-O3` isn't coming from an explicitly set `CMAKE_CXX_FLAGS_RELEASE`.
With this PR:
```
$ cmake -B build -DCMAKE_BUILD_TYPE=Release -DCMAKE_CXX_FLAGS_RELEASE="-O3"
<snip>
C++ compiler flags .................... -O3 -std=c++20 -fPIC -fno-extended-identifiers -fdebug-prefix-map=/home/hebasto/dev/bitcoin/src=. -fmacro-prefix-map=/home/hebasto/dev/bitcoin/src=. -fstack-reuse=none -Wall -Wextra -Wformat -Wformat-security -Wvla -Wredundant-decls -Wdate-time -Wduplicated-branches -Wduplicated-cond -Wlogical-op -Woverloaded-virtual -Wsuggest-override -Wimplicit-fallthrough -Wunreachable-code -Wbidi-chars=any -Wundef -Wno-unused-parameter -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=3 -Wstack-protector -fstack-protector-all -fcf-protection=full -fstack-clash-protection
Linker flags .......................... -O3 -fstack-reuse=none -fstack-protector-all -fcf-protection=full -fstack-clash-protection -Wl,-z,relro -Wl,-z,now -Wl,-z,separate-code -fPIE -pie
```
and
```
$ cmake -B build -DCMAKE_BUILD_TYPE=Release
<snip>
C++ compiler flags .................... -O2 -std=c++20 -fPIC -fno-extended-identifiers -fdebug-prefix-map=/home/hebasto/dev/bitcoin/src=. -fmacro-prefix-map=/home/hebasto/dev/bitcoin/src=. -fstack-reuse=none -Wall -Wextra -Wformat -Wformat-security -Wvla -Wredundant-decls -Wdate-time -Wduplicated-branches -Wduplicated-cond -Wlogical-op -Woverloaded-virtual -Wsuggest-override -Wimplicit-fallthrough -Wunreachable-code -Wbidi-chars=any -Wundef -Wno-unused-parameter -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=3 -Wstack-protector -fstack-protector-all -fcf-protection=full -fstack-clash-protection
Linker flags .......................... -O2 -fstack-reuse=none -fstack-protector-all -fcf-protection=full -fstack-clash-protection -Wl,-z,relro -Wl,-z,now -Wl,-z,separate-code -fPIE -pie
```
When calling `cmake` repeatedly using the same build directory, each newly provided `CMAKE_CXX_FLAGS_RELEASE` value will be accommodated. In such a scenario, if the user wishes to revert to the build system defaults, they should unset the `CMAKE_CXX_FLAGS_RELEASE` variable by passing `-UCMAKE_CXX_FLAGS_RELEASE` to `cmake`.
---
This PR does not aim to resolve _all_ issues mentioned in https://github.com/bitcoin/bitcoin/issues/31491.
ACKs for top commit:
purpleKarrot:
ACK edde96376a
janb84:
ACK [edde963](edde96376a)
ryanofsky:
Code review ACK edde96376a
Tree-SHA512: 1fbc879bd02cf0be726ced490f65985e728f0686ccb3a32cd38787b56377aa666e1965448e5069515abc814df49a0083c8000bc3f6f322f5f395695638168fb6
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