CMake: Replace CMAKE_COMPILER_IS_GNUCC with CMAKE_C_COMPILER_ID

CMAKE_COMPILER_IS_GNUCC is obsolete variable and can be replaced with
CMAKE_C_COMPILER_ID (also available since early CMake versions).

In the past CMake versions also LCC and QCC compilers had this varible
set to boolean true but these aren't relevant here.

cmake/sdlcompilers.cmake

@@ -11,7 +11,7 @@ macro(SDL_DetectCompiler)
     if(MSVC)
       set(MSVC_CLANG TRUE)
     endif()
-  elseif(CMAKE_COMPILER_IS_GNUCC)
+  elseif(CMAKE_C_COMPILER_ID STREQUAL "GNU")
     set(USE_GCC TRUE)
   elseif(CMAKE_C_COMPILER_ID MATCHES "^Intel$")
     set(USE_INTELCC TRUE)