I'll add a note to the build instructions. That said, libpng shouldn't be necessary. The Armory script uses its own binary, as I recall. Something must've failed during the build process. (Then again, I wouldn't mind moving some of this over to brew. Will think about it.)
I had some issues myself for awhile. IIRC, goatpig & I determined that there was some sort of fcgi issue. I believe we meant to look into it again once 0.96 was ready. I haven't had any problems, and honestly, I've been dealing with some family issues, which has been eating up a lot of my time. I'll take another look as soon as I can, though. (Will be gone this weekend, so it may be a little while.)