Fix setting cflags/libs from sdl2-config

SDL_CFLAGS and SDL_LIBS assignment was "only if absent". However due
to previously assigning them to "pkg-config sdl2" values, the values
from sdl2-config were ignored.

Makefile

@@ -301,8 +301,8 @@ endif
 # Use sdl2-config if all else fails
 ifeq ($(SDL_CFLAGS),)
   ifneq ($(call bin_path, sdl2-config),)
-    SDL_CFLAGS ?= $(shell sdl2-config --cflags)
-    SDL_LIBS ?= $(shell sdl2-config --libs)
+    SDL_CFLAGS = $(shell sdl2-config --cflags)
+    SDL_LIBS = $(shell sdl2-config --libs)
   endif
 endif