Merge bitcoin/bitcoin#32356: cmake: Respect user-provided configuration-specific flags

edde963 cmake: Respect user-provided configuration-specific flags (Hennadii Stepanov)

Pull request description:

  This PR addresses [this](bitcoin/bitcoin#31491 (comment)) 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 bitcoin/bitcoin#31491.

ACKs for top commit:
  purpleKarrot:
    ACK edde963
  janb84:
    ACK [edde963](bitcoin/bitcoin@edde963)
  ryanofsky:
    Code review ACK edde963

Tree-SHA512: 1fbc879bd02cf0be726ced490f65985e728f0686ccb3a32cd38787b56377aa666e1965448e5069515abc814df49a0083c8000bc3f6f322f5f395695638168fb6

Author: ryanofsky

CMakeLists.txt

@@ -19,6 +19,10 @@ if(POLICY CMP0171)
   cmake_policy(SET CMP0171 NEW)
 endif()
 
+# When adjusting CMake flag variables, we must not override those explicitly
+# set by the user. These are a subset of the CACHE_VARIABLES property.
+get_directory_property(precious_variables CACHE_VARIABLES)
+
 #=============================
 # Project / Package metadata
 #=============================

cmake/module/ProcessConfigurations.cmake

@@ -105,14 +105,13 @@ function(remove_cxx_flag_from_all_configs flag)
 endfunction()
 
 function(replace_cxx_flag_in_config config old_flag new_flag)
-  string(TOUPPER "${config}" config_uppercase)
-  string(REGEX REPLACE "(^| )${old_flag}( |$)" "\\1${new_flag}\\2" new_flags "${CMAKE_CXX_FLAGS_${config_uppercase}}")
-  set(CMAKE_CXX_FLAGS_${config_uppercase} "${new_flags}" PARENT_SCOPE)
-  set(CMAKE_CXX_FLAGS_${config_uppercase} "${new_flags}"
-    CACHE STRING
-    "Flags used by the CXX compiler during ${config_uppercase} builds."
-    FORCE
-  )
+  string(TOUPPER "CMAKE_CXX_FLAGS_${config}" var_name)
+  if("${var_name}" IN_LIST precious_variables)
+    return()
+  endif()
+  string(REGEX REPLACE "(^| )${old_flag}( |$)" "\\1${new_flag}\\2" ${var_name} "${${var_name}}")
+  set(${var_name} "${${var_name}}" PARENT_SCOPE)
+  set_property(CACHE ${var_name} PROPERTY VALUE "${${var_name}}")
 endfunction()
 
 set_default_config(RelWithDebInfo)