[PATCH] configure: fix mktemp call for macOS

[David Bremner]

Ralph Seichter <abbot at monksofcool.net> writes:

> The mktemp call in configure is incompatible with /usr/bin/mktemp as
> shipped with macOS, where it fails due to a missing parameter. I added
> the missing template, and the call is now working with both Apple's
> and GNU coreutils' mktemp.
>

I'm hoping Tomi will comment, but I wonder if we should do something
like

TEMP_GPG=$(mktemp -d "${TMPDIR:-/tmp}/notmuch.XXXXXX")

(borrowed from test/test-lib.sh). This avoids hardcoding the temporary
directory.