Only execute llvm-config when needed, previously it was being executed always in
order to set PYTHONPATH.  For example, running "make clean" required llvm-config
even though it made no use of the output.


git-svn-id: https://llvm.org/svn/llvm-project/dragonegg/trunk@171744 91177308-0d34-0410-b5e6-96231b3b80d8