Add support for 64 bits time_t

[danieljakots]

trafficstars

See #101 for details.

[jcourreges]

Hi there, jca@openbsd here, new maintainer of our pqiv port.

A few comments on @danieljakots' proposal:

• g_date_time_to_unix() returns a gint64. To print it portably without a cast one should use glib's G_GINT64_FORMAT.
• Turns out that pqiv.c tries to avoid G_GINT64_FORMAT on WIN32, cf pqiv.c line 3710. Using %jd and an (intmax_t) cast supposedly works around the problem (%jd is the correct format for a signed integer).
• Except, you seem to have hit a problem with mingw builds and the use of %ju, and have switched to PRIuMAX. Care to share what was the problem that lead to this commit?

[phillipberndt]

Sorry for ignoring this for so long, I have fallen quite behind on PRs. Thanks for the comment, too!

The MingW issue was simply that it didn't support "j" at the time. I don't bother to check right now and will just add the conditional win32 check to this branch, too.