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>

Makefile

@@ -488,6 +488,7 @@ pathsep = :
 bindir_relative = $(patsubst $(prefix)/%,%,$(bindir))
 mandir_relative = $(patsubst $(prefix)/%,%,$(mandir))
 infodir_relative = $(patsubst $(prefix)/%,%,$(infodir))
+gitexecdir_relative = $(patsubst $(prefix)/%,%,$(gitexecdir))
 htmldir_relative = $(patsubst $(prefix)/%,%,$(htmldir))
 
 export prefix bindir sharedir sysconfdir gitwebdir perllibdir localedir
@@ -1735,6 +1736,7 @@ infodir_relative_SQ = $(subst ','\'',$(infodir_relative))
 perllibdir_SQ = $(subst ','\'',$(perllibdir))
 localedir_SQ = $(subst ','\'',$(localedir))
 gitexecdir_SQ = $(subst ','\'',$(gitexecdir))
+gitexecdir_relative_SQ = $(subst ','\'',$(gitexecdir_relative))
 template_dir_SQ = $(subst ','\'',$(template_dir))
 htmldir_relative_SQ = $(subst ','\'',$(htmldir_relative))
 prefix_SQ = $(subst ','\'',$(prefix))