RFR: Allow using a system installed libpng

[Omair Majid]

Hi,

The following webrevs modify the build system to allow building against
the system-installed copy of libpng as well as using the bundled copy of
libpng

ROOT: http://cr.openjdk.java.net/~omajid/webrevs/system-libpng/01/
JDK: http://cr.openjdk.java.net/~omajid/webrevs/system-libpng/01-jdk/

A new configure option `--with-libpng` is introduced which defaults to
`bundled` but can be set to `system` to use the system-installed copy of
libpng.

This patch (with minor differences) has been used in Fedora builds for a
while now [1].

Thanks,
Omair

[1] http://pkgs.fedoraproject.org/cgit/java-1.8.0-openjdk.git/commit/system-libpng.patch?id=ff9254ee03799a98689692ee9ae060b3a31af213

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