Fix issue #64: Allow build outside of clang source tree
CMake configuration now allows out-of-tree builds if user specifies -DLLVM_PATH=... pointing to path of LLVM/Clang install directory.
Based on patch by Christopher Berner.

git-svn-id: http://include-what-you-use.googlecode.com/svn/trunk@466 812d1f90-f944-d88a-411a-122bdc93e1ea