@migueldeicaza says "The problem is that we installed a pkg-config
program in /usr/bin which only looked up at the Mono database of
packages, as it was a symlink to the one in
/Library/Frameworks/Mono.framework This caused problem for people that
either had their own, or were using popular Unix install systems like
Homebrew or MacPorts. The correct fix is to detect if
exists, if it does, use that, instead use /usr/bin/pkg-config"