Fix shell and patch bugs breaking nokogiri.

Hi! So I've been able to track a couple of serious bugs preventing nokogiri from building to you're code here.
One is that you're using 'sh', but the packages hardcode `/bin/sh`; if there's another `sh` in the path; and it has a different set of features, your 'configuring' with one shell and making with the other, so it breaks.

Secondly, you're `which` code doesn't seem to walk the path right over directories with whitespace. So I've replaced it with something much simpler and effective.

Lastly, Mac OS X no longer has any version of GCC, just a wrapper for clang that uses some really old gcc headers. So I've changed the fallback compiler to `cc` system compiler.

Best,
G