diff --git a/package/exim/exim.mk b/package/exim/exim.mk
index c07d6f919c..0ad9a8de80 100644
--- a/package/exim/exim.mk
+++ b/package/exim/exim.mk
@@ -127,7 +127,8 @@ define EXIM_BUILD_CMDS
 		LNCC=$(HOSTCC) \
 		CFLAGS="-std=c99 $(HOST_CFLAGS)" \
 		LFLAGS="-fPIC $(HOST_LDFLAGS)"
-	$(TARGET_MAKE_ENV) build=br $(MAKE1) -C $(@D) $(EXIM_STATIC_FLAGS)
+	$(TARGET_MAKE_ENV) build=br $(MAKE1) -C $(@D) $(EXIM_STATIC_FLAGS) \
+		CFLAGS="-std=c99 $(TARGET_CFLAGS)"
 endef
 
 # Need to replicate the LFLAGS in install, as exim still wants to build