clover: conditionally use MESA_GIT_SHA1

clover: conditionally use MESA_GIT_SHA1

Considering how hard/annoying it was for many peoples' workflow to
properly generate the macro, it will be demoted to conditionally
available with follow-up commits.

v2: Kill off gracious blank line (Vedran).

Cc: mesa-stable@lists.freedesktop.org
Cc: Vedran Miletić <vedran@miletic.net>
Cc: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com> (v1)
Reviewed-by: Vedran Miletić <vedran@miletic.net>