50c50,51
< CFLAGS = -O -Wall $(INCS) $(MINGW_CCFLAGS)
---
> CPPFLAGS = $(INCS)
> CFLAGS = -O -Wall $(MINGW_CCFLAGS)
88c89
< 	$(CC) -c $(CFLAGS) $<
---
> 	$(CC) -c $(CPPFLAGS) $(CFLAGS) $<
91c92
< 	$(CC) -c $(CFLAGS) -DPNG_BUILD_DLL -o $@ $<
---
> 	$(CC) -c $(CPPFLAGS) $(CFLAGS) -DPNG_BUILD_DLL -o $@ $<