improve libwebp (#781)

* Add external source for libwebp

* improve libwebp

Co-authored-by: Kelvin Zhang <zhangxp1998@gmail.com>

packages/l/libpng/xmake.lua

@@ -17,7 +17,7 @@ package("libpng")
         add_syslinks("m")
     end
 
-    on_install("windows", "mingw", "android", "iphoneos", "cross", function (package)
+    on_install("windows", "mingw", "android", "iphoneos", "cross", "bsd", function (package)
         io.writefile("xmake.lua", [[
             add_rules("mode.debug", "mode.release")
             add_requires("zlib")

packages/l/libwebp/xmake.lua

@@ -22,6 +22,8 @@ package("libwebp")
     if is_plat("linux", "bsd") then
         add_syslinks("pthread")
     end
+    add_extsources("brew::webp")
+    add_extsources("apt::libwebp-dev")
 
     on_install("linux", "macosx", "windows", "mingw", "bsd", function (package)
         local configs = {}