added support for release branches of clang and llvm via git. fixes #3

fixes issue #3 that I created earlier.
Added clang 3.4 - mingw64 needs 3.4 so that's why I'm doing this early ;)
Seems to not break anything and just git pull's to update as we get closer and closer to 3.4 tag which is due 23rd December
We can change is to CT_GetFile after llvm site add's it

Martell