diff --git a/mosquitto.spec b/mosquitto.spec
index 3a0e1a9..afe4ebd 100644
--- a/mosquitto.spec
+++ b/mosquitto.spec
@@ -46,7 +46,11 @@ sed -i "s|prefix?=/usr/local|prefix?=/usr|" config.mk
 sed -i "s|(INSTALL) -s|(INSTALL)|g" lib/Makefile src/Makefile client/Makefile
 
 %build
+%if 0%{?rhel} == 7
+export CFLAGS="%{optflags} -std=gnu99"
+%else
 export CFLAGS="%{optflags}"
+%endif
 export LDFLAGS="%{optflags} %{__global_ldflags} -Wl,--as-needed"
 make all %{?_smp_mflags} WITH_WEBSOCKETS=yes WITH_SYSTEMD=yes WITH_BUNDLED_DEPS=no