AC_SYS_LARGEFILE now widens time_t if possible

AC_SYS_LARGEFILE now widens time_t if possible

This change is taken from Gnulib, and is needed for apps like GDB.
* lib/autoconf/specific.m4 (_AC_SYS_YEAR2038_TEST_INCLUDES)
(_AC_SYS_YEAR2038, AC_SYS_YEAR2038): New macros, taken (with
renaming) from Gnulib.
(_AC_SYS_LARGEFILE_MACRO_VALUE): #undef before #define.
(AC_SYS_LARGEFILE): Prefer AS_IF and AS_CASE to doing it by hand.
Widen time_t if possible, too.  Define  __MINGW_USE_VC2005_COMPAT
early if needed.