Skip test of AC_SYS_YEAR2038_RECOMMENDED if system doesn’t support 64-bit time_t.

Skip test of AC_SYS_YEAR2038_RECOMMENDED if system doesn’t support 64-bit time_t.

AC_SYS_YEAR2038_RECOMMENDED causes configure to fail if we can’t find
a way to activate support for 64-bit time_t.  This makes its
mktests.pl test fail on systems that don’t support 64-bit time_t.
Switch to a hand-written test that’s skipped on those systems.
While I was in there I added slightly more testing of the closely
related macros AC_SYS_LARGEFILE and AC_SYS_YEAR2038.

While testing this change I noticed that AC_SYS_LARGEFILE and
AC_SYS_YEAR2038 malfunction if AC_LANG([C++]) is in effect.
The cause is too hairy to fix before the release; add a mention
to NEWS instead.  (Bug <https://savannah.gnu.org/support/index.php?110983>.)

* tests/mktests.pl: Exclude AC_SYS_LARGEFILE, AC_SYS_YEAR2038, and
  AC_SYS_YEAR2038_RECOMMENDED from automatic test generation.
* tests/semantics.at: Add manual tests of those macros.