gmtl: Patch scons script to build on clang

The upstream scons script only allows to compile using gcc, but not clang, so gmtl was failing to build on OS X 10.9.4, [here is my output](https://gist.github.com/anonymous/640e7ba3bea1c0f9fe72)

I've added a patch to the formula that fixes this issue. Unfortunately upstream gmtl seems unresponsive (last answered bug in http://sourceforge.net/p/ggt/bugs/ dates from March 2011), so I didn't submit the patch there.