Fix POSIX only support for _get_local_utc_offset()

[freach]

The usage of the "tm_gmtoff" field in tm struct (src/rfc3339.c) is currently only supported by POSIX systems. For cross-platform compatibility fallbacks should be offered.

[Delgan]

I read this in time module documentation:

Changed in version 3.6: The struct_time attributes tm_gmtoff and tm_zone are now available on all platforms.

Could that help with that issue?