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 <email@example.com> Signed-off-by: Junio C Hamano <firstname.lastname@example.org>