Remove `time_t` from API (bug #61821). The bit-width of `time_t` might change depending on compile time switches. That can cause a mismatch between class declarations in public headers and the actual alignment of the compiled classes. Avoid that problem by not using `time_t` in (public) headers. * build-aux/mk-octave-config-h.sh, oct-conf-post.in.h: Define OCTAVE_TIME_T to `int64_t`. The type of `time_t` is not defined in the C/C++ standards. However in all(?) implementations, it's either a signed 32-bit or a signed 64-bit integer. Use a signed 64-bit integer for storing those values in Octave classes. * other files: Use OCTAVE_TIME_T instead of `time_t`. Don't include <ctime> in files where it's not needed.