[luatex] Problem building 0.50.0 (and trunk)

[Karl Berry]

            if which bash >/dev/null

I doubt "which" is sufficiently portable, given that we're already
talking about systems that don't have /bin/bash.

There is no really easy way to do this.  Checking along PATH has its own
pain.  (; vs. :, cygwin vs. COMSPEC, .exe vs. x bits, etc., etc.  See
the texi2dvi script for gory details.)

Hope the actual CONFIG_SHELL-related fixes can be installed in luatex
easily enough and make the whole issue go away.