Add support to build using Hombrew on Mac OS X

Omit compiling strlcpy and strlcat on Mac OS X due to compilation errors:
```
In file included from calmwm.c:36:
./calmwm.h:28:8: error: conflicting types for '__builtin___strlcat_chk'
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/usr/include/secure/_string.h:111:3: note:
      expanded from macro 'strlcat'
  __builtin___strlcat_chk (dest, src, len, __darwin_obsz (dest))
```

I welcome feedback on how to improve the proposed changes.