Documentation: Fix man page breakage with DocBook XSL v1.72

Documentation: Fix man page breakage with DocBook XSL v1.72

From version 1.72 it will replace all dots in roff requests with U+2302
("house" character), and add escaping in output for all instances of dot
that are not in roff requests. This caused the ".ft" hack forcing
monospace font in listingblocks to end up as "\&.ft" and being visible
in the resulting man page.

The fix adds a DOCBOOK_XSL_172 build variable that will disable the
hack. To allow this variable to be defined in config.mak it also moves
build variable handling below the inclusion of config.mak.

Signed-off-by: Jonas Fonseca <fonseca@diku.dk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>