Use pkg-config in Makefile.simple.

Let's see if TravisCI is happy with this.
Addresses, but does not fully resolve, #135.

Author: Quuxplusone

Makefile.simple

@@ -48,7 +48,7 @@ ifeq (${OS},linux)
   LDFLAGS_GL := -lGL
   LDFLAGS_GLEW := -lGLEW
   LDFLAGS_PNG := -lpng
-  LDFLAGS_SDL := -lSDL -lSDL_gfx -lSDL_mixer -lSDL_ttf -lpthread -lz
+  LDFLAGS_SDL := $(shell pkg-config --libs sdl SDL_gfx SDL_mixer SDL_ttf) -pthread -lz
   OBJ_EXTENSION := .o
   hyper_RES :=
 endif
@@ -74,7 +74,7 @@ ifeq (${OS},osx)
   LDFLAGS_GL := -framework AppKit -framework OpenGL
   LDFLAGS_GLEW := -lGLEW
   LDFLAGS_PNG := -lpng
-  LDFLAGS_SDL := -lSDL -lSDLMain -lSDL_gfx -lSDL_mixer -lSDL_ttf -lpthread -lz
+  LDFLAGS_SDL := $(shell pkg-config --libs sdl SDL_gfx SDL_mixer SDL_ttf) -pthread -lz
   OBJ_EXTENSION := .o
   hyper_RES :=
 endif