improve libpng

packages/l/libpng/xmake.lua

@@ -22,7 +22,7 @@ package("libpng")
     on_install("windows", function (package)
         local configs = {"-DPNG_TESTS=OFF",
                          "-DPNG_SHARED=OFF", 
-                         "-DPNG_DEBUG=" .. package:debug() and "ON" or "OFF"}
+                         "-DPNG_DEBUG=" .. (package:debug() and "ON" or "OFF")}
         import("package.tools.cmake").install(package, configs)
     end)