build: simplify and make more portable to non-GNU make

build: simplify and make more portable to non-GNU make

The AC_SUBST'd variable '$(NO_INSTALL_PROGS_DEFAULT)' is only used in
makefile expressions expanding the list of manual pages that are not
built by default (but might need to be when a distribution tarball
is created). Such expressions exploited a feature of make variable
expansion -- namely, $(VAR:%=dir/%.x) -- that, while seemingly quite
portable in practice, is not POSIX-conforming, and could break on
lesser vendor make implementations.  So kill two birds with one stone,
by getting rid of the $(NO_INSTALL_PROGS_DEFAULT) intermediate variable
and improving makefile portability in the process.

While at it, we also clean up some other minor naming inconsistency
and useless indirection.

* configure.ac (NO_INSTALL_PROGS_DEFAULT): Don't define or AC_SUBST
anymore; instead ...
(EXTRA_MANS): ... define and AC_SUBST these.
* man/local.mk (extra_man_1): Rename ...
(EXTRA_MANS): ... like this, explicitly making clear it's AC_SUBST'd.
(extra_man_x): It's used only once, no need to define it; just inline
its only expansion where needed.
(EXTRA_DIST): Adjust.
(ALL_MANS): New, union of $(EXTRA_MANS) and $(dist_man1_MANS).
* cfg.mk (check-x-vs-1, sc_option_desc_uppercase): Rely on $(ALL_MANS)
rather than on $(NO_INSTALL_PROGS_DEFAULT) and $(dist_man1_MANS).