add NO_EXTERNAL_GREP build option

add NO_EXTERNAL_GREP build option

Previously, we just chose whether to allow external grep
based on the __unix__ define. However, there are systems
which define this macro but which have an inferior group
(e.g., one that does not support all options used by t7002).
This allows users to accept the potential speed penalty to
get a more consistent grep experience (and to pass the
testsuite).

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>