Allow using a system installed libpng

A lot of Linux distributions prefer that programs use the system-installed version of libraries instead of bundling libraries. OpenJDK currently bundles a copy of libpng and uses that to build.

It would be nice if the build system could be modified to allow building against the system installed copy of libpng as well as the bundled copy.
                                                                                                                                                      
Mailing list thread with a patch and discussion
http://mail.openjdk.java.net/pipermail/build-dev/2014-February/011717.html