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.

With this, you can now run tests from a terminal on Mac like so:

```
$ make MONO_PKG_CONFIG=/Library/Frameworks/Mono.framework/Commands/pkg-config test
```
Change MONO_PKG_CONFIG to PKG_CONFIG and note usage in README
Merge branch 'add-make-var-for-pkg-config' of https://github.com/michaelbartnett/litjson into michaelbartnett-add-make-var-for-pkg-config
Getting ready pull request #27
Merge branch 'michaelbartnett-add-make-var-for-pkg-config'