Support --with-llvmgccdir and friends in llvmc, take 2.

Should now work when building with objdir != srcdir and when llvm-gcc is not
available.

Thanks to Duncan Sands for testing and advice!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@69700 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed