diff --git a/package/exim/exim.mk b/package/exim/exim.mk
index 118ee4a1a4..1f814ef195 100644
--- a/package/exim/exim.mk
+++ b/package/exim/exim.mk
@@ -125,7 +125,7 @@ define EXIM_BUILD_CMDS
 	$(HOST_MAKE_ENV) $(MAKE1) -C $(@D)/build-br macro_predef \
 		CC=$(HOSTCC) \
 		LNCC=$(HOSTCC) \
-		CFLAGS="$(HOST_CFLAGS)" \
+		CFLAGS="-std=c99 $(HOST_CFLAGS)" \
 		LFLAGS="-fPIC $(HOST_LDFLAGS)"
 	$(TARGET_MAKE_ENV) build=br $(MAKE1) -C $(@D) $(EXIM_STATIC_FLAGS)
 endef