maint: fix build on newer glibc.

POSIX allows, but does not require, that <fcntl.h> implicitly
includes <sys/stat.h>.  Until recent glibc, most systems did
this implicit inclusion, and m4 was mistakenly relying on it.
* m4/output.c (includes): Add <sys/stat.h>.
* THANKS: Update.
Reported by Todd Rinaldo.

Signed-off-by: Eric Blake <eblake@redhat.com>
maint: enable additional c++ tests with make dist.

* configure.ac (gl_CXX_CHOICE_DEFAULT_NO): Make default for C++
tests be disabled...
* Makefile.am (DISTCHECK_CONFIGURE_FLAGS): ...but run them when
building package.
* ltdl/m4/gnulib-cache.m4: Regenerate.

Signed-off-by: Eric Blake <eblake@redhat.com>
maint: add recent copyright assignments.

* AUTHORS: Add Joel Denny and David Warme.

Signed-off-by: Eric Blake <eblake@redhat.com>
texinfo: drop obsolete @acronym invocations.

* doc/m4.texi: Remove all uses of @acronym.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>