diff --git a/package/exim/exim.mk b/package/exim/exim.mk
index 0ad9a8de80..e4b0bfc1b4 100644
--- a/package/exim/exim.mk
+++ b/package/exim/exim.mk
@@ -135,7 +135,9 @@ endef
 # something when installing...
 define EXIM_INSTALL_TARGET_CMDS
 	DESTDIR=$(TARGET_DIR) INSTALL_ARG="-no_chown -no_symlink" build=br \
-	  $(MAKE1) -C $(@D) $(EXIM_STATIC_FLAGS) install
+	  $(MAKE1) -C $(@D) $(EXIM_STATIC_FLAGS) \
+		CFLAGS="-std=c99 $(TARGET_CFLAGS)" \
+		install
 	chmod u+s $(TARGET_DIR)/usr/sbin/exim
 endef