Makefile: do not export mandir/htmldir/infodir

These are defined in the main Makefile to be funny values that are optionally relative to an unspecified location that is determined at runtime. They are only suitable for hardcoding in the binary via the -DGIT_{MAN,HTML,INFO}_PATH=<value> C preprocessor options, and are not real paths, contrary to what any sane person, and more importantly, the Makefile in the documentation directory, would expect. A longer term fix is to introduce runtime_{man,html,info}dir variables to hold these funny values, and make {man,html,info}dir variables to have real paths whose default values begin with $(prefix), but as a first step, stop exporting them from the top-level Makefile Signed-off-by: Junio C Hamano <gitster@pobox.com>