cmake: convert spaces in PTHREAD_CFLAGS to ';'

cmake/sdlchecks.cmake

@@ -814,6 +814,7 @@ macro(CheckPTHREAD)
       }" HAVE_PTHREADS)
     if(HAVE_PTHREADS)
       set(SDL_THREAD_PTHREAD 1)
+      separate_arguments(PTHREAD_CFLAGS)
       target_compile_options(sdl-build-options INTERFACE ${PTHREAD_CFLAGS})
       list(APPEND SDL_EXTRA_LDFLAGS ${PTHREAD_LDFLAGS})
       list(APPEND SDL_PC_CFLAGS ${PTHREAD_CFLAGS})