Most Autoconf configure scripts don't support the non-standard `--with-gcc`
flag, and there does not seem to be any good reason to do so, since
Autoconf has already a proper way to set the c-compiler:

Either via the `CC` environment variable, or (with higher precedence)
by passing a commandline argument of the form `CC=clang-3.8`.

This patch simply replaces the previous non-standard `--with-gcc=clang-3.8`
argument (which was clearly misnamed to begin with) by a `CC=clang-3.8`
command-line argument to the `configure` script.