Add support to find libresolv on Homebrew/Mac OS X

When building tinc 1.0.24 on Mac OS X using [Homebrew](http://brew.sh) the compile fails due to:

```
Undefined symbols for architecture x86_64:
  "_res_9_init", referenced from:
      _main_loop in net.o
```

since `-lresolv` is not passed to the linker.

This pull request adds changes with which libresolv is found when installing tinc with homebrew.

I have only tested these changes on Mac OS X and welcome feedback for improvement.