32bit time_t fixes.

Fixes #35

First commit defines the alias scm_to/from_time_t (maybe this should have a gnutls prefix instead of the guile prefix but idk) to the correct size integer conversion function and errors if it is not 4 or 8. Second commit is a bit of hack to get the SIZEOF_TIME_T value into the scheme test via an environment variable and restores the previous test values in case it is 32 bit. Don't know if there is a better to achieve the same result.

I built and ran the tests successfully on a 32bit machine and with guix build -f guix.scm -s i686-linux (apart from the SRP test.)