diff --git a/Makefile.am b/Makefile.am
index 41fcc66..f281879 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -244,7 +244,7 @@ EXTRA_DIST = \
 endif
 
 # Options for the tests
-tests_cflags = -I$(srcdir)/tests $(AM_CFLAGS)
+tests_cflags = -I$(srcdir)/tests $(AM_CFLAGS) -std=gnu99
 tests_ldadd = libcortex-strings.la
 tests_test_memchr_LDADD = $(tests_ldadd)
 tests_test_memchr_CFLAGS = $(tests_cflags)