Self-contained clang/libc++/LLVM installation

In case somebody finds it helpful, I put a script together that builds
a self-contained clang/libc++/LLVM combo, installed under a given
directory prefix independent of any existing system-wide compiler:

    https://github.com/rsmmr/install-clang

The script currently supports Darwin (using libabi++) and Linux (using
libcxxrt), and applies a few small patches before building the source
code to adjust search paths. This hasn't been tested much yet but
seems to work for me; ymmv.

See the README for more. Suggestions for improvements/simplifications
welcome.

Robin