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

[elaexuotee]

CONFIG_SHELL simply acts as a user override; it's not part of autoconf's core
logic. That role belongs to the SHELL macro, who's picks the first available of
the following:

- CONFIG_SHELL environment variable,
- SHELL environment variable, or
- /bin/sh

See autoconf's m4sugar/m4sh.m4 for the gory details. Arguably, this should
also be updated to point to a fixed /bin/sh output fallback.

Anyway, AM_SUBST_NOTMAKE([SHELL]), cf. '(automake) Optional', simply tells
automake to not define SHELL inside the generated Makefile. This means that
make will instead use it's default, which in our case is hard-coded to the
/bin/sh in its implicit bash-minimal dependency. For detailed info about this
behaviour of make, see '(make) Choosing the Shell'. Note, however, you will
have to do a mental sed-replace of "/bin/sh" with "<bash-minimal>/bin/sh" when
reading that page.


Cheers,
B.