maint: assume long long int

maint: assume long long int

It’s long been safe to assume C99+ support for long long int.
* .gitignore: Remove m4/longlong.m4.
* bootstrap.conf (buildreq): Boost git prereq from 1.4.4 to 1.5.5,
syncing with Gnulib.
(bootstrap_post_import_hook): Remove m4/longlong.m4.
* m4/jm-macros.m4 (gl_CHECK_ALL_TYPES):
No need to require AC_TYPE_UNSIGNED_LONG_LONG_INT.
* src/factor.c (DItype, UDItype):
* src/od.c (main):
Assume HAVE_LONG_LONG_INT.
* src/od.c: (unsigned_long_long_int):
Remove.  All uses replaced with unsigned long long int.