build: use pkg-config for SDL2

SDL2 hasn't required suing sdl2-config in a long time and using
pkg-config can have better results for distros.

projects/unix/Makefile

@@ -351,16 +351,18 @@ endif
 
 # test for presence of SDL
 ifeq ($(origin SDL_CFLAGS) $(origin SDL_LDLIBS), undefined undefined)
-  SDL_CONFIG = $(CROSS_COMPILE)sdl2-config
-  ifeq ($(shell which $(SDL_CONFIG) 2>/dev/null),)
-    $(error No SDL development libraries found!)
-  else
-    ifeq ($(NETPLAY), 1)
-      SDL_LDLIBS += -lSDL2_net
+  ifeq ($(shell $(PKG_CONFIG) --modversion sdl2 2>/dev/null),)
+    $(error No SDL2 development libraries found!)
+  endif
+  ifeq ($(NETPLAY), 1)
+    ifeq ($(shell $(PKG_CONFIG) --modversion SDL2_net 2>/dev/null),)
+      $(error No SDL2_net development libraries found!)
     endif
+    SDL_CFLAGS += $(shell $(PKG_CONFIG) --cflags SDL2_net)
+    SDL_LDLIBS += $(shell $(PKG_CONFIG) --libs SDL2_net)
   endif
-  SDL_CFLAGS  += $(shell $(SDL_CONFIG) --cflags)
-  SDL_LDLIBS += $(shell $(SDL_CONFIG) --libs)
+  SDL_CFLAGS += $(shell $(PKG_CONFIG) --cflags sdl2)
+  SDL_LDLIBS += $(shell $(PKG_CONFIG) --libs sdl2)
 endif
 CFLAGS += $(SDL_CFLAGS)
 LDLIBS += $(SDL_LDLIBS)