I posted recently about XQuartz causing compile errors related to libpng. The problem was likely not due to a problem in XQuartz, but the installation triggered a mismatch in libpng versions. I didn't have any user-installed libpng files when the errors occurred, and the cause was a mystery. My only solution was to install my own libpng into /usr/local and prune out the ones in /opt/X11. This is what I have been doing for the last few revs. Each time I installed XQuartz the problem came back until I pruned them out. I want to report that XQuartz 2.6.1_rc1 does not have this problem. I installed it and repeated my test of compiling an X11 app that requires libpng. I expected errors, and expected to prune out /opt/X11 again. But this time the apps compiled without error. Did something change in XQuartz 2.6.1_rc1 with regards to libpng? .pc files? Whatever it was, it's working now.