Add environment variable check to ignore macports/fink warning.

Backstory:
I run Homebrew, MacPorts, and Fink.  They (so far) haven't cause any problems, and each provided packages that the others didn't have.  I don't really have any intention of removing them at this point, since nothing has gone wrong yet, and every time Homebrew installs something it yells at me for it.

Solution:
I added a case in the check_macports function in order to turn off displaying the warning if an environment variable is set to anything non-empty.  This is only hit when installing a package, and not when using 'brew doctor'.  This way I can squelch that pesky warning with one line in my ~/.bashrc.

Other thoughts:
I picked an environment variable because it seemed like the easiest "permanent" solution, and I was also unsure where to document this.