config.mak.in: allow "configure --sysconfdir=/else/where"

We do allow vanilla Makefile users to say make sysconfdir=/else/where
and config.mak can also be tweaked manually for the same effect. Give
the same configurablity to ./configure users as well.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

config.mak.in

@@ -18,6 +18,7 @@ bindir = @bindir@
 gitexecdir = @libexecdir@/git-core
 datarootdir = @datarootdir@
 template_dir = @datadir@/git-core/templates
+sysconfdir = @sysconfdir@
 
 mandir=@mandir@