Actually check for uint64_t.
* configure.ac: Check size of uint64_t and the UINT64_C macro.
configure.ac used $ac_cv_sizeof_uint64_t but never set this variable.
Due to the availability of long long on all platforms supporting
uint64_t this was not a real problem. Found while remove the
corresponding test from gnupg.
- Signed-off-by: Werner Koch <wk@gnupg.org>