Always include non-configurable part of CPPFLAGS

Makefile

@@ -6,7 +6,7 @@ all: sxiv
 
 include config.mk
 
-CPPFLAGS += -I. -DVERSION=\"$(VERSION)\" -DHAVE_GIFLIB=$(HAVE_GIFLIB) -DHAVE_LIBEXIF=$(HAVE_LIBEXIF)
+override CPPFLAGS += -I. -DVERSION=\"$(VERSION)\" -DHAVE_GIFLIB=$(HAVE_GIFLIB) -DHAVE_LIBEXIF=$(HAVE_LIBEXIF)
 
 LDLIBS := -lImlib2 -lX11 -lXft