Makefile: add a gitexecdir_relative variable

Makefile: add a gitexecdir_relative variable

This variable will be e.g. "libexec/git-core" if
gitexecdir=/tmp/git/libexec/git-core is given. It'll be used by a
subsequent change.

This is stolen from the yet-to-be integrated (needs resubmission)
"Makefile: add Perl runtime prefix support" patch on the mailing
list. See
<20180108030239.92036-3-dnj@google.com> (https://public-inbox.org/git/20180108030239.92036-3-dnj@google.com/).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>