date.c: Add hard-coded definition of HAVE_TIMEZONE

The original code expected this to be set by running configure.
We'll just manually set it here for now. This isn't as portable
as if we were doing some compile-time examination of the current
system, but I don't need portability now.

When someone comes along that wants to port notmuch to another
system, they will already have all the #ifdefs in place and
will simply need to add the appropriate machinery to set the
defines.