[PATCH] configure: fix mktemp call for macOS

[Ralph Seichter]

* David Bremner:

> TEMP_GPG=$(mktemp -d "${TMPDIR:-/tmp}/notmuch.XXXXXX")
>
> (borrowed from test/test-lib.sh). This avoids hardcoding the temporary
> directory.

I had the same thought at first, but I want to ensure that the path
length does not cause trouble anymore.

  > uname -v
  Darwin Kernel Version 18.6.0: Thu Apr 25 23:16:27 PDT 2019; root:xnu-4903.261.4~2/RELEASE_X86_64
  > echo $TMPDIR
  /var/folders/m_/w5lk3r_1495dhnl4l0s96pr00000gn/T/

That's my MacBook Pro with macOS 10.14.5 (Mojave). AFAIK, older versions
can have longer names. Also, I am concerned with accumulating clutter.

  > grep clean_tmps_dirs /etc/defaults/periodic.conf
  daily_clean_tmps_dirs="/tmp"

The /tmp directory will, with default settings, be cleaned periodically
by the OS, while $TMPDIR (which is user-specific) will not.

-Ralph