There are a bunch of tests that check for time_t handling 64-bit values
properly. This makes sense, but also obviously doesn't work when time_t
is 32-bit, and causes compile-time errors. Compile these tests out in
that case.
Since there's no straightforward way to check sizeof(time_t) inside the
proprocessor, only do this when glibc's __TIMESIZE is defined.
That's what this class really is; in fact that's what the first line of
the comment says it is.
This commit does not rename the main files, since those will contain
other time-related classes in a little bit.
