[PATCH] configure: fix mktemp call for macOS

[Ralph Seichter]

* Tomi Ollila:

> First, it would be better if
>
> TEMP_GPG=$(mktemp -d "${TMPDIR:-/tmp}/notmuch.XXXXXX")
>
> worked, then the behaviour on Linux would not change.

I'm trying to figure out if I can override $TMPDIR during the MacPorts
configure phase. According to the documentation, modifying environment
variables should work, but I am not quite there yet.

Corollary: I suggest we go for the method shown above for the time
being. I am not a fan of hardcoding paths either.

-Ralph