Add pic variant for libpng (#37964)

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

@@ -37,6 +37,7 @@ class Libpng(CMakePackage):
         multi=True,
         description="Build shared libs, static libs or both",
     )
+    variant("pic", default=False, description="PIC")
 
     @property
     def libs(self):
@@ -53,6 +54,7 @@ def cmake_args(self):
             self.define("ZLIB_ROOT", self.spec["zlib-api"].prefix),
             self.define("PNG_SHARED", "shared" in self.spec.variants["libs"].value),
             self.define("PNG_STATIC", "static" in self.spec.variants["libs"].value),
+            self.define_from_variant("CMAKE_POSITION_INDEPENDENT_CODE", "pic"),
         ]
         if self.spec.satisfies("platform=darwin target=aarch64:"):
             args.append("-DPNG_ARM_NEON=off")