apply part of https://github.com/Homebrew/linuxbrew/pull/64

As discussed in issue #164 , this patch is a simplified version of pull request #64 which skirts X11 dependencies on linux.

The original patch seems to be trying to validate X11 installation by checking the `xterm --version`, but
it doesn't seem to be finished. This patch just assumes you have X11 installed if you are on linux.

This seems like an acceptable holdover until something along the lines of #64 is figured out.