cmake: collect SDL_LIBS as list + export PKG_DIRECTFB_LDFLAGS

cmake/sdlchecks.cmake

@@ -733,6 +733,7 @@ macro(CheckDirectFB)
       set(SDL_VIDEO_DRIVER_DIRECTFB 1)
       set(SDL_VIDEO_RENDER_DIRECTFB 1)
       list(APPEND EXTRA_CFLAGS ${PKG_DIRECTFB_CFLAGS})
+      list(APPEND SDL_CFLAGS ${PKG_DIRECTFB_CFLAGS})
       if(SDL_DIRECTFB_SHARED AND NOT HAVE_SDL_LOADSO)
         message_warn("You must have SDL_LoadObject() support for dynamic DirectFB loading")
       endif()
@@ -773,7 +774,7 @@ macro(CheckVivante)
         find_library(VIVANTE_VDK_LIBRARY VDK REQUIRED)
         list(APPEND EXTRA_LIBS ${VIVANTE_LIBRARY} ${VIVANTE_VDK_LIBRARY})
       else()
-        set(SDL_CFLAGS "${SDL_CFLAGS} -DLINUX -DEGL_API_FB")
+        list(APPEND SDL_CFLAGS -DLINUX -DEGL_API_FB)
         list(APPEND EXTRA_LIBS EGL)
       endif(HAVE_VIVANTE_VDK)
     endif()
@@ -921,8 +922,7 @@ macro(CheckPTHREAD)
       set(SDL_THREAD_PTHREAD 1)
       list(APPEND EXTRA_CFLAGS ${PTHREAD_CFLAGS})
       list(APPEND EXTRA_LDFLAGS ${PTHREAD_LDFLAGS})
-      set(SDL_CFLAGS "${SDL_CFLAGS} ${PTHREAD_CFLAGS}")
-      list(APPEND SDL_LIBS ${PTHREAD_LDFLAGS})
+      list(APPEND SDL_CFLAGS ${PTHREAD_CFLAGS})
 
       check_c_source_compiles("
         #define _GNU_SOURCE 1