diff --git a/src/Makefile.am b/src/Makefile.am
index a9382d45..04ac0edf 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -1,4 +1,4 @@
-AM_CFLAGS = -g -O2 -Wall -Werror -Wno-deprecated-declarations -fno-strict-aliasing
+AM_CFLAGS = -g -O2 -Wall -Werror -Wno-deprecated-declarations -fno-strict-aliasing -std=c99
 AM_CFLAGS += -I$(top_builddir)/libev
 AM_CFLAGS += -I$(top_builddir)/libasyncns
 
diff --git a/src/Makefile.in b/src/Makefile.in
index b09433ee..fe0ba831 100644
--- a/src/Makefile.in
+++ b/src/Makefile.in
@@ -249,7 +249,7 @@ top_build_prefix = @top_build_prefix@
 top_builddir = @top_builddir@
 top_srcdir = @top_srcdir@
 AM_CFLAGS = -g -O2 -Wall -Werror -Wno-deprecated-declarations \
-	-fno-strict-aliasing -I$(top_builddir)/libev \
+	-fno-strict-aliasing -std=c99 -I$(top_builddir)/libev \
 	-I$(top_builddir)/libasyncns
 AM_LDFLAGS = -static
 SS_COMMON_LIBS = $(top_builddir)/libev/libev.la $(INET_NTOP_LIB)