gnat 2012 (compiler + IDE)

Hello,

I started working on a formula to install the GNAT Ada compiler.
It may not be ready yet, but I supposed a pull request was a good way to get some feedback...

The formula works for me, but I still have the following issues:

- It also installs the IDE, because I couldn't find a way to download it separately... Should I move it to a separate formula, or is it ok ?
- It also installs a custom gcc version (since GNAT is gcc-based) in /usr/local/bin, which may conflict with both MacOS version and other formulas... Is it ok, or should I try to build it from sources using the system gcc ? Or maybe not installing it to /usr/local/bin and adding PATH configuration instructions (which is what their install script does actually) ?

I also plan to add formulas for other libraries soon (ASIS, AUnit, GNATColl, SPARK and XMLAda).

Any comment ?