mapserver buildbot configure flags

[thomas bonfort]

hi
I've added png quantization for rgba images in the mapserver trunk
(http://trac.osgeo.org/mapserver/ticket/2436). could you possibly
update the buildbot ./configure ( Tamas: and also very probably
nmake.opt in trunk, I have no idea what would be needed on windows )
so this functionality can be included in the tests (at least to check
for a clean build)? it's activated with --with-experimental-png

thanks,
thomas