Update clang-redist to also build Clang on OS X.

There's no point in building libstdc++ on OS X.  Someone who wants a C++11
toolchain on OS X can get one, easily, by installing the Xcode command-line
tools, which includes a recent Clang and libc++.  The command-line tools
are not sufficient for the SourceWeb indexer, though, which needs a
specific version of Clang's static libraries.

The SourceWeb indexer can *almost* use the Clang packages provided by
llvm.org, but that package is built against libstdc++, and SourceWeb needs
to use libc++ for C++11 support.

Notes:

 * Ubuntu 10.04 has both sha256sum (a C program) and shasum (a Perl
   script).  OS X only has shasum.

 * OS X has curl, but does not have wget.