Detect LLVM version to properly find Clang std headers.
Detect LLVM version to properly find Clang std headers.

We only require version 3.4.x, so assuming that the user is using
3.4(.0) would lead to not finding the std headers when running the clang
tool.

Change-Id: I1bab89eb0f1d50705eefc42f10bb921c4e9c94ad

rgly/LLVM-Tracer,rgly/LLVM-Tracer,rgly/LLVM-Tracer