CMake: Only use -fno-rtti for C++

Using it for C generates the following warning:
command line option '-fno-rtti' is valid for C++/D/ObjC++ but not for C

build/cmake/init.cmake

@@ -96,7 +96,7 @@ if(wxUSE_NO_RTTI)
     if(MSVC)
         add_compile_options("/GR-")
     elseif(("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU") OR ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang"))
-        add_compile_options("-fno-rtti")
+        wx_string_append(CMAKE_CXX_FLAGS " -fno-rtti")
     endif()
     add_definitions("-DwxNO_RTTI")
 endif()