diff --git a/lib/Makefile.am b/lib/Makefile.am
index 6b75ba4..0e31a45 100644
--- a/lib/Makefile.am
+++ b/lib/Makefile.am
@@ -20,4 +20,4 @@ include gnulib.mk
 noinst_HEADERS += cmpbuf.h prepargs.h
 libdiffutils_a_SOURCES += cmpbuf.c prepargs.c
 
-AM_CFLAGS = $(GNULIB_WARN_CFLAGS) $(WERROR_CFLAGS)
+AM_CFLAGS += $(GNULIB_WARN_CFLAGS) $(WERROR_CFLAGS)