libpng package: find correct zlib library on Windows (#49034)

This explicitly specifies the correct library for zlib to CMake:

CMake's find zlib looks for zlib before zdll. On Windows, zlib is the
static lib, and zdll the import library. LibPNG only links to shared
zlib, but was getting zlib from CMake on Windows, which was resulting
in a linker failure.

var/spack/repos/builtin/packages/libpng/package.py

@@ -62,6 +62,9 @@ def cmake_args(self):
             self.define("PNG_STATIC", "static" in self.spec.variants["libs"].value),
             self.define_from_variant("CMAKE_POSITION_INDEPENDENT_CODE", "pic"),
         ]
+        zlib_lib = self.spec["zlib-api"].libs
+        if zlib_lib:
+            args.append(self.define("ZLIB_LIBRARY", zlib_lib[0]))
         if self.spec.satisfies("platform=darwin target=aarch64:"):
             args.append("-DPNG_ARM_NEON=off")
         return args