Core/Buildsystem: Clean up compileflags when compiling with warnings fully enabled.

-Wwrite-strings is default enabled when -Wall is set in clang/gcc, hence removed.
 -Woverloaded-virtual is now enabled for C++-code only, this removes the useless warnings when dabbling with C-only code.

cmake/compiler/gcc/settings.cmake

@@ -11,9 +11,9 @@ add_definitions(-DHAVE_SSE2 -D__SSE2__)
 message(STATUS "GCC: SFMT enabled, SSE2 flags forced")
 
 if( WITH_WARNINGS )
-  set(WARNING_FLAGS "-W -Wall -Wextra -Wwrite-strings -Winit-self -Woverloaded-virtual -Winvalid-pch -Wfatal-errors")
+  set(WARNING_FLAGS "-W -Wall -Wextra -Winit-self -Winvalid-pch -Wfatal-errors")
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${WARNING_FLAGS}")
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${WARNING_FLAGS}")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${WARNING_FLAGS} -Woverloaded-virtual")
   message(STATUS "GCC: All warnings enabled")
 endif()