change to build on Windows

Makefile

@@ -1,11 +1,15 @@
-.PHONY: default
-default:
-	@rake $@
+ifeq ($(OS),Windows_NT)
+	GL_LIB := opengl32
+else
+	GL_LIB := GL
+endif
+CFLAGS := $(shell freetype-config --cflags) $(shell sdl2-config --cflags)
+CXXFLAGS := $(CFLAGS)
+LDFLAGS := -l$(GL_LIB) -ldl -lfreetype $(shell sdl2-config --libs) $(shell freetype-config --libs)
 
-.PHONY: test
-test:
-	@rake $@
+freetype2gl3: gl3w.o freetype2gl3.o
+	$(CXX) -o $@ $^ $(LDFLAGS)
 
 .PHONY: clean
 clean:
-	@rake $@
+	-rm -f freetype2gl3