diff --git a/mail/exim/Makefile b/mail/exim/Makefile
index e848ff29e2..7e7906ac84 100644
--- a/mail/exim/Makefile
+++ b/mail/exim/Makefile
@@ -137,6 +137,7 @@ define Package/exim/conffiles
 /etc/exim/exim.conf
 endef
 
+HOST_CFLAGS += -std=c99
 TARGET_CFLAGS += $(FPIC) -DNO_IP_OPTIONS -D_FILE_OFFSET_BITS=64
 
 MAKE_VARS += build=Linux-$$(ARCH)