longlong: tune, particularly for common case of c99

longlong: tune, particularly for common case of c99

* m4/longlong.m4 (AC_TYPE_LONG_LONG_INT): Don't bother compiling
or running anything if c99, or if unsigned long long int does not
work.  In either case, we know the answer without further tests.
Do not compile _AC_TYPE_LONG_LONG_SNIPPET twice.  Instead, compile
it at most once, and use its results for both long long int and
unsigned long long int.  This is more likely to be efficient in
the common case where the program wants to check for both long
long int and unsigned long long int.
(AC_TYPE_UNSIGNED_LONG_LONG_INT): Don't bother compiling if c99,
since the answer is already known.