Make sure that externally provided CXX flags are not discarded.

Mostly the ones coming from the CXXFLAGS environment variable,
defined in special builds of the project.

cmake/configure_cpp_options.cmake

@@ -21,12 +21,12 @@ set( CMAKE_CXX_EXTENSIONS FALSE CACHE BOOL "(Dis)allow using GNU extensions" )
 set( CMAKE_CXX_STANDARD_REQUIRED TRUE CACHE BOOL "Require the specified C++ standard for the build" )
 
 # Setting CMAKE_CXX_FLAGS to avoid "deprecated" warnings
-set(CMAKE_CXX_FLAGS "-Wno-deprecated-declarations" ) # very basic
-#set(CMAKE_CXX_FLAGS "-Wall -Werror -pedantic-errors -Wno-deprecated-declarations" ) # good enough for a quick, better check
-#set(CMAKE_CXX_FLAGS "-Wall -Wextra -Werror -pedantic-errors -Wno-deprecated-declarations" ) # better for a thorough check
-#set(CMAKE_CXX_FLAGS "-Wall -Wextra -Werror -pedantic-errors" ) # better for an even more severe check
-#set(CMAKE_CXX_FLAGS "-Weverything -Werror -pedantic-errors" ) # not recommended, it warns for really EVERYTHING!
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-deprecated-declarations" ) # very basic
+#set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Werror -pedantic-errors -Wno-deprecated-declarations" ) # good enough for a quick, better check
+#set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Werror -pedantic-errors -Wno-deprecated-declarations" ) # better for a thorough check
+#set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Werror -pedantic-errors" ) # better for an even more severe check
+#set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Weverything -Werror -pedantic-errors" ) # not recommended, it warns for really EVERYTHING!
 
 # TODO: for Debug and with GCC, do we want to set the flags below by default?
 # set( CMAKE_BUILD_TYPE DEBUG )
-# set(CMAKE_CXX_FLAGS "-fPIC -O0 -g -gdwarf-2" )
+# set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fPIC -O0 -g -gdwarf-2" )