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>