libpng: use spdx licensing method

lib/licenses.nix

@@ -182,6 +182,11 @@ rec {
     fullName = "GNU Lesser General Public License v3.0 or later";
   };
 
+  libpng = spdx {
+    shortName = "Libpng";
+    fullName = "libpng License";
+  };
+
   libtiff = {
     shortName = "libtiff";
     fullName = "libtiff license";
@@ -292,4 +297,3 @@ rec {
   };
 
 }
-

pkgs/development/libraries/libpng/12.nix

@@ -26,6 +26,7 @@ stdenv.mkDerivation rec {
   meta = {
     description = "The official reference implementation for the PNG file format";
     homepage = http://www.libpng.org/pub/png/libpng.html;
-    license = "free-non-copyleft"; # http://www.libpng.org/pub/png/src/libpng-LICENSE.txt
+    license = stdenv.lib.licenses.libpng;
+    maintainers = with stdenv.lib.maintainers; [ fuuzetsu ];
   };
 }

pkgs/development/libraries/libpng/15.nix

@@ -19,6 +19,7 @@ stdenv.mkDerivation rec {
   meta = {
     description = "The official reference implementation for the PNG file format";
     homepage = http://www.libpng.org/pub/png/libpng.html;
-    license = "free-non-copyleft"; # http://www.libpng.org/pub/png/src/libpng-LICENSE.txt
+    license = stdenv.lib.licenses.libpng;
+    maintainers = with stdenv.lib.maintainers; [ fuuzetsu ];
   };
 }

pkgs/development/libraries/libpng/default.nix

@@ -30,8 +30,8 @@ in stdenv.mkDerivation rec {
   meta = with stdenv.lib; {
     description = "The official reference implementation for the PNG file format" + whenPatched " with animation patch";
     homepage = http://www.libpng.org/pub/png/libpng.html;
-    license = "free-non-copyleft"; # http://www.libpng.org/pub/png/src/libpng-LICENSE.txt
+    license = licenses.libpng;
     platforms = platforms.all;
-    maintainers = [ maintainers.vcunat ];
+    maintainers = [ maintainers.vcunat maintainers.fuuzetsu ];
   };
 }