RFR(xs): 8172712: configure should check that grep handles empty pattern correctly

[Thomas Stüfe]

Dear all,

please take a look at this fix. This should prevent configure from using a
grep which is not able to handle pattern lists with empty patterns.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8172712

webrevs:
http://cr.openjdk.java.net/~stuefe/webrevs/8172712-configure-should-check-that-grep-handles-empty-pattern-correctly/webrev.00/webrev/

Background:

https://bugs.openjdk.java.net/browse/JDK-8172579 was a build error caused
by a buggy grep (on AIX) and the easiest way to work around that bug would
be to use the GNU version of grep. It was suggested in the orginal mail
thread (
http://mail.openjdk.java.net/pipermail/build-dev/2017-January/018495.html)
to check this in configure.

This change implements this suggestion and checks if grep behaves correctly.

Kind Regards, Thomas