Allow --enable/disable-year2038 to be set in environment.

In both AC_SYS_YEAR2038 and AC_SYS_YEAR2038_RECOMMENDED, only set
the shell variable ‘enable_year2038’ if it doesn’t already have a
value, thus allowing it to be passed down from the environment.

Fixes <https://savannah.gnu.org/support/?111394>.

* lib/autoconf/specific.m4 (_AC_SYS_YEAR2038_ENABLE): When setting
  the default value of enable_year2038, do not override a value that
  was set in the environment.
* tests/semantics.at: Add tests for setting enable_year2038 in the
  environment and in config.site.