build libpng on windows

packages/l/libpng/xmake.lua

@@ -3,7 +3,8 @@ package("libpng")
     set_homepage("http://www.libpng.org/pub/png/libpng.html")
     set_description("The official PNG reference library")
 
-    set_urls("https://github.com/glennrp/libpng/archive/$(version).zip")
+    set_urls("https://github.com/glennrp/libpng/archive/$(version).zip",
+             "https://github.com/glennrp/libpng.git")
     add_versions("v1.6.35", "3d22d46c566b1761a0e15ea397589b3a5f36ac09b7c785382e6470156c04247f")
     add_versions("v1.6.34", "7ffa5eb8f9f3ed23cf107042e5fec28699718916668bbce48b968600475208d3")