cmake: Respect user-provided configuration-specific flags

Author: hebasto

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)