build: don't use recursive make to build the 'src' subdirectory

build: don't use recursive make to build the 'src' subdirectory

* Makefile.am (SUBDIRS): Remove 'src'.  Ensure '.' is listed before
'tests' and 'gnulib-tests'.
(dist-hook): Adjust: we must now tweak the top-level Makefile.in
in $(distdir), not the one in the 'src/' subdir (which is gone).
(include): The '$(top_srcdir)/src/local.mk' file.
* build-aux/gen-lists-of-programs.sh: Adjust the generation of the
automake input fragment.
* tests/Makefile.am (.built-programs): Adjust.
* cfg.mk (all_programs): Remove this convenience rule; it's no
longer needed, now that we can rely directly on the contents of
$(all_programs).
(sc_option_desc_uppercase, check-programs-vs-x:): Adjust lists
of prerequisites accordingly.
(all-progs-but-lbracket): Simplify definition accordingly.
* configure.ac ($OPTIONAL_BIN_PROGS): Adjust definition.
($OPTIONAL_PKGLIB_PROGS): Likewise.
($NO_INSTALL_PROGS_DEFAULT): Tweak definition, for consistency.
(AC_CONFIG_FILES): Remove 'src/Makefile'.
* src/Makefile.am: Rename ...
* src/local.mk: ... like this, with a lot of adjustments.  In
addition ...
(all_programs): ... remove this now-unneeded convenience target.