Discover a POSIX shell for Solaris

Under Solaris, use ksh instead of `/bin/sh`.

This uses `$SHELL` as a POSIX shell, coupled with a `configure` check
that sets it correctly.

Note that the POSIX shell might end up being bash, so this actually
introduces more fragmentation than it reduces.

Taken from https://github.com/freedreno/mesa/blob/master/configure.ac