configure: use compliant grep regex checks

The current `grep "foo\|bar"' trips on some grep implementations, like
the FreeBSD one. Instead use `egrep "foo|bar"' as suggested by Stefan.

Cc: Stefan Esser <se@FreeBSD.org>
Reported-by: Stefan Esser <se@FreeBSD.org>
Bugzilla: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=228673


Fixes: 1914c814 ("configure: error out if building OMX w/o supported platform")
Fixes: 63e11ac2 ("configure: error out if building VA w/o supported platform")
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
(cherry picked from commit dfb1f275)