Fix #107 - Installing on Mono 3.0.4

See https://github.com/fsharp/fsharp/issues/107

@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 /Library/Frameworks/Mono.framework/Versions/Current/bin/pkg-config exists, if it does, use that, instead use /usr/bin/pkg-config"