diff --git a/unix/fbiterm/src/Makefile.am b/unix/fbiterm/src/Makefile.am
index 4ffe3da..53f12f4 100644
--- a/unix/fbiterm/src/Makefile.am
+++ b/unix/fbiterm/src/Makefile.am
@@ -17,7 +17,7 @@ fbiterm_SOURCES = \
 fbiterm_LDADD = \
 	-L$(libdir) @X_LIBS@ -lm -lXfont -literm -lfreetype
 
-CFLAGS = -I$(includedir) @X_CFLAGS@ -I$(top_srcdir)/include -I/usr/local/include $(EXTRA_CFLAGS)
+CFLAGS += -I$(includedir) @X_CFLAGS@ -I$(top_srcdir)/include -I/usr/local/include $(EXTRA_CFLAGS)
 
 install-exec-hook:
 	chmod u+s $(DESTDIR)$(bindir)/fbiterm