Add make variable for specifying pkg-config

Mono ships its own pkg-config which knows about all of its tools, such as nunit-console2. It doesn't really play nice with Mac and Homebrew, so many leave it out of their path.

This change lets you run tests from a terminal on Mac like so:

```
$ make MONO_PKG_CONFIG=/Library/Frameworks/Mono.framework/Commands/pkg-config test
```