Honor configure's htmldir switch

Honor autoconf's --htmldir switch. This allows relocating HTML docs
straight from the configure script.
Signed-off-by: NChristoph J. Thompson <cjsthompson@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

config.mak.in

@@ -20,7 +20,8 @@ datarootdir = @datarootdir@
 template_dir = @datadir@/git-core/templates
 sysconfdir = @sysconfdir@
 
-mandir=@mandir@
+mandir = @mandir@
+htmldir = @htmldir@
 
 srcdir = @srcdir@
 VPATH = @srcdir@