Changed configure to construct a Makefile that builds on Mac OS X

elvis builds OK on Mac OS X (with X11) with just a small change...
uname will discover the system name as "darwin" and build it as a generic Unix which works just fine with one exception: gcc needs to have -L<X11_LIB_DIR> specified. 
My workaround is to force the use of --x-libraries= and then use that specified value in the -L argument. If --x-libraries= is not specified and this is to be an X11 build configure exit()s with a message to use that option. If this isn't an X11 build then nothing changes.
Attached is a screenshot of elvis running on Mac OS X.
![elvis_os-x](https://f.cloud.github.com/assets/380945/296263/3e1770f8-94eb-11e2-9607-fa9618f36011.png)