Get rid of "grep empty(sub)expression" noise.

In OS X (and assumedly *BSD) Configure output starts with:

grep empty(sub)expression

apparently because plain 'grep' understands the 'os\(/\|\)2'
somewhat differently (BRE vs ERE, RTFM re_format(7))

Cure: redirect the stderr of grep to stdout (which is then promptly
redirected to /dev/null).  The grep will still fail, and not take
the OS/2 (or DJGPP) branch.