maint: move maintainer make rules in maint.mk This is done for two reasons: 1. It will enable us to take advantage of more GNU make features in our maintainer-specific recipes, if we want to (and we probably will). 2. The Makefile.am won't have to be modified each time a maintainer rule needs to be changed or tweaked; this will prevent useless remaking of (in particular) the 't/testsuite-part.am' file. * maint.mk: New, several recipes, rules and variables for maintainer-specific tasks, extracted (with some tweaks) from ... * Makefile.am: ... this file. (EXTRA_DIST): Add 'maint.mk'. * GNUmakefile: Include 'maint.mk' after 'Makefile'. Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>