Allow INSTALL, bindir, mandir to be set in main Makefile

Allow INSTALL, bindir, mandir to be set in main Makefile

Makefiles in subdirectories now use existing value of INSTALL, bindir,
mandir if it is set, allowing those to be set in main Makefile or in
included config.mak.  Main Makefile exports variables which it sets.

Accidentally it renames bin to bindir in Documentation/Makefile
(should be bindir from start, but is unused, perhaps to be removed).

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>