Change Makefile to add to CFLAGS instead of replacing them.
--- a/Makefile
+++ b/Makefile
@@ -18,7 +18,7 @@
 
 CPPFLAGS+=	`${PKG_CONFIG} --cflags x11 xft xrandr`
 
-CFLAGS?=	-Wall -O2 -g -D_GNU_SOURCE
+CFLAGS+=	-Wall -O2 -g -D_GNU_SOURCE
 
 LDFLAGS+=	`${PKG_CONFIG} --libs x11 xft xrandr`
 
