[PATCH v3] configure.ac: Avoid empty subexpression in grep

[Paul Eggert]

Alexey Neyman wrote:
>>
>>> replace all the /dev/null redirects with a -q flag.
>>
>> That also works.
>>
> Version 3 of the patch attached.

Hmm, sorry, I forgot about this issue:

https://bugs.gnu.org/24941

The problem is that in many cases

    FOO | grep -q BAR

will cause FOO to output a "Broken pipe" diagnostic, whereas

    FOO | grep BAR >/dev/null

will be quiet (and also less efficient, but nobody cares about performance 
here). So perhaps we'd better avoid the /dev/null-to-minus-q part of the change, 
and stick to just fixing the bug at hand for now.