Substitution of SHELL and CONFIG_SHELL

[Niels Möller]

When running configure generated by autoconf-2.59 on Solaris,
CONFIG_SHELL gets set to /bin/bash, which is ok. But then the value
somehow leaks over to SHELL, resulting in

  SHELL=/bin/bash

being substituted into all my Makefiles. Is there a good reason for
that? Rules in Makefiles are generally written for /bin/sh, and unlike
configure, speed and availability of $LINENO doesn't really matter.

So I would generally feel a lot more comfortable with SHELL=/bin/sh in
the Makefiles.

I noticed this because only because one user couldn't got bash errors
when running make, and it turned out he had a set -u in his .bashrc.
To me, doing that unconditionally in .bashrc seems like a very broken
thing to do, but the problem would be avoided if make used plain
SHELL=/bin/sh to execute the rules, the way $DEITY intended it to do.

Regards,
/Niels