bug#40641: Building from git breaks when /bin/sh isn't bash

[elaexuotee]

> I think we’re not on the same page.  Is AM_SUBST_NOTMAKE([SHELL]) really
> problematic?  Is seems like there is a legitimate use-case that foreign
> distro users with /bin/sh = dash would want “guix shell -D guix -- make”
> to just work without workaround?  We could use elaexuotee’s
> AM_SUBST_NOTMAKE([SHELL]) patch, could we not?

Just to be clear, that CONFIG_SHELL workaround was a giant pain to figure out.
The errors produced when /bin/sh = dash are not at all obvious as to the root
cause, and finding the existence of CONFIG_SHELL takes a certain familiarity
with autoconf, which I didn't have until significant manual reading.

Seems pretty draconian to leave that for foreign distro devs if we do indeed
have a good fix.