Extra comment pointing to boost bug report
Now using PATH_TO_LLVM_ROOT

This is the first step towards eliminating the in-tree copy of llvm.
Removing the in-tree copy of llvm+clang
Removing support for static linking to libclang
ClangCompleter tests not built when no libclang

When the user doesn't want semantic completion with clang, then tests that test
those features are not included in the test binary.