diff --git a/Makefile.am b/Makefile.am
index e49b7c59..cf67c4ba 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -27,7 +27,7 @@ AM_CPPFLAGS += -DDATADIR='"$(datadir)"' -DLIBEXECDIR='"$(libexecdir)"' \
 	-DG_LOG_DOMAIN=\"OSTree\" \
 	-DGLIB_VERSION_MIN_REQUIRED=GLIB_VERSION_2_40 -DGLIB_VERSION_MAX_ALLOWED=GLIB_VERSION_2_40 \
 	-DSOUP_VERSION_MIN_REQUIRED=SOUP_VERSION_2_40 -DSOUP_VERSION_MAX_ALLOWED=SOUP_VERSION_2_48
-AM_CFLAGS += $(WARN_CFLAGS)
+AM_CFLAGS += -std=gnu99 $(WARN_CFLAGS)
 DISTCHECK_CONFIGURE_FLAGS += --enable-gtk-doc --disable-maintainer-mode
 
 GITIGNOREFILES = aclocal.m4 build-aux/ buildutil/*.m4 config.h.in gtk-doc.make