Fix for 'ld: library not found for -lXau' error in mtools

`brew install mtools` gave me the following error:
```
==> Downloading ftp://ftpmirror.gnu.org/mtools/mtools-4.0.17.tar.gz
Already downloaded: /Library/Caches/Homebrew/mtools-4.0.17.tar.gz
==> ./configure LIBS=-liconv --prefix=/usr/local/Cellar/mtools/4.0.17
==> make install
/usr/local/Cellar/mtools/4.0.17/bin/mbadblocks
/usr/local/Cellar/mtools/4.0.17/bin/mzip
ld: library not found for -lXau
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [floppyd] Error 1

READ THIS: https://github.com/mxcl/homebrew/wiki/troubleshooting
```

With the attached patch it installs successfully.