[PATCH] configure: add set -u

[David Bremner]

Tomi Ollila <tomi.ollila at iki.fi> writes:

> In case of any unset variable, make ./configure exit with nonzero value;
> an attempt to expand an unset variable is a bug in the script
> (usually a spelling mistake) and those should not pass through
> unnoticed.
> ---
>
> I was also doing set -e support but that has quite a few nontrivial
> things in it. Set -u catches all lagging c++ compilers and so on ;).
> Perhaps later.

This commit has an unintended consequence: on unknown platforms,
configure crashes and doesn't generate Makefile.config at all. In
particular this manifests on Debian/kfreebsd but I imagine there are
other platforms where people are just ignoring the "Notmuch might or
might not build correctly".  I'm tempted to revert the commit for now,
and give ourselves some time after the release of 0.23 to figure out
(and test) the best solution.

d