Fix #107 - Installing on Mono 3.0.4

@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"
Merge pull request #109 from funnelweb/fix107

Fix #107 - Installing on Mono 3.0.4