[PATCH v2] configure: fix out of tree build; check unsafe characters in srcdir

[David Bremner]

Tomi Ollila <tomi.ollila at iki.fi> writes:
> To my surprise mkdir 'build dir'; cd 'build dir'; ../notmuch/configure
> and then `make` just worked. :D
>
> I did not dare to run tests -- I'm pretty sure that would fail...
>

20-ish tests currently fail looking for json_check_nodes.py

But yeah, many other tests explode if the out of tree directory has a
space in it.

d