Makefile now makes use of the `-s <shell>` option for Urchin, IF it is available, for true cross-shell testing.
Conversely, a warning is issued if it is missing, because that implies that no actual cross-shell testing will take place.

Also, the shell command that determines the set of available test suites is now POSIX-compliant.
Merge pull request #568 from mklement0/fix_make_file

Fixed Urchin invocation problem in zsh and dash.  Makefile "future-proofed" to take advantage of true cross-shell testing with Urchin, once available.