Allow using a system-installed giflib

[Omair Majid]

Hi,

I have a webrev at:
http://cr.openjdk.java.net/~omajid/webrevs/system-giflib/00/

It introduces a configure option --with-giflib that (similar to the
existing --with-zlib) allows specifying whether the build should use the
system installed giflib or the one bundled with OpenJDK.

I have not tested it extensively yet (just verified that it builds fine
with both configurations), but I will do that over the coming days if
the patch looks (somewhat) acceptable for OpenJDK8.

Thanks,
Omair
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