diff --git a/alsaloop/Makefile.am b/alsaloop/Makefile.am
index f76eafd..a694911 100644
--- a/alsaloop/Makefile.am
+++ b/alsaloop/Makefile.am
@@ -1,6 +1,6 @@
 INCLUDES = -I$(top_srcdir)/include
 LDADD = -lm
-CFLAGS += -D_GNU_SOURCE
+AM_CFLAGS = -D_GNU_SOURCE
 if HAVE_SAMPLERATE
 LDADD += -lsamplerate
 endif