configure: disallow whitespace in paths, extend checks to $PWD

configure: disallow whitespace in paths, extend checks to $PWD

Whitespace in $NOTMUCH_SRCDIR (and $PWD) may work in builds,
but definitely will not work in tests. It would be difficult
to make tests support whitespace in test filename paths -- and
fragile to maintain if done.

So it is just easier and safer to disallow whitespace there.

In case of out of tree build $NOTMUCH_SRCDIR differs from $PWD
(current directory). Extend this whitespace, and also previously
made unsafe characters check to $PWD too.