contribs: Fix iconv build when using gcc 5.x

GCC 5.x defaults to -std=gnu11, which changes the __inline keyword
semantics, causing some iconv functions not to be inlined, but they are
not declared as exported, thus causing linking to iconv to fail

contrib/src/iconv/rules.mak

@@ -33,6 +33,6 @@ endif
 	$(MOVE)
 
 .iconv: iconv
-	cd $< && $(HOSTVARS) ./configure $(HOSTCONF) --disable-nls
+	cd $< && $(HOSTVARS) ./configure CFLAGS="$(CFLAGS) -fgnu89-inline" $(HOSTCONF) --disable-nls
 	cd $< && $(MAKE) install
 	touch $@