SunOS grep does not understand -C<n> nor -e

SunOS grep does not understand -C<n> nor -e

The first "grep -C1" test in t7002 does not pass on my SunOS-5.11-i86pc,
and that is not because our way to spawn external grep is broken, but
because the native grep does not understand -C<n>.

It turns out that Peff was also using this option himself because our
Makefile doesn't do that automatically.  Brandon Casey uses SUNWspro
compiler without having to set this, and it turns out that the compiler
does not define preprocessor macro __unix__ which made him always use the
built-in grep, never an external one.

Let's be more explicit and say that we do not use external grep on Suns.

Signed-off-by: Junio C Hamano <gitster@pobox.com>