AC_TYPE_LONG_LONG_INT busted in current Autoconf

Hi,

In recent versions of Autoconf, AC_TYPE_LONG_LONG_INT (and
AC_TYPE_UNSIGNED_LONG_LONG_INT) wrongly indicate that (unsigned) long
long is supported on compilers which actually do not support it.

Looking at the implementation of AC_TYPE_LONG_LONG_INT, it contains:

  ac_cv_type_long_long_int=yes
  case $ac_prog_cc_stdc in
  no | c89) ;;
  *) [ run compiler probes ]
  esac

which looks like it is assuming "long long" is supported for
pre-standard and C89 compilers, only actually running checks
on compilers which support newer C standards; presumably this
is the opposite of what is intended.

The failure is easy to demonstrate with gcc:

  % cat >configure.ac <<'EOF
  AC_INIT([test], [0])
  AC_PROG_CC
  AC_TYPE_LONG_LONG_INT
  AC_OUTPUT
EOF
  
  % autoconf-2.72 --force
  % ./configure CC='gcc -Werror=long-long'
  [...]
  checking for gcc -Werror=long-long option to enable C11 features... unsupported
  checking for gcc -Werror=long-long option to enable C99 features... unsupported
  checking for gcc -Werror=long-long option to enable C89 features... none needed
  checking for unsigned long long int... yes
  checking for long long int... yes

The last apparently working version is autoconf-2.70, but that is
because this version of autoconf has a different bug which causes it to
believe C89 compilers support C99 (so the long long test is run in this
version).  The last actually working version is autoconf-2.69:

  % autoconf-2.69 --force
  % ./configure CC='gcc -Werror=long-long'
  [...]
  checking for gcc -Werror=long-long option to accept ISO C89... none needed
  checking for unsigned long long int... no
  checking for long long int... no

Cheers,
  Nick