build: fix generation of manpages for programs not built by default
* src/local.mk (dist-hook): Don't use this to ensure all the
programs, even the ones disabled by default or by the user, are
built (doing so is required to ensure the distributed manpages
are properly built). This would build those programs too late
anyway, causing errors like:
$ make dist
make dist-xz am__post_remove_distdir='@:'
make[1]: Entering directory `~/src/coreutils'
GEN man/arch.1
help2man: can't get '--help' info from man/arch.td/arch
make[1]: *** [man/arch.1] Error 127
make[1]: Leaving directory `~/src/coreutils'
make: *** [dist] Error 2
Instead, ...
* man/local.mk (extra_man_x, extra_man_1): define these ...
($(extra_man_1)): ... and make this depend on $(all_programs).
(EXTRA_DIST): Adjust.
Adjust some comments as well.