AIX compilation failed, because glibc's non-standard extension to
`struct tm` were used - additional members tm_gmtoff and tm_zone.

The patch fixes it by adding corresponding compile-time check.

Additionally, for the calculation of GMT offset on AIX, a portable
variant of timegm() was required.Implementation here is inspired by
SergeyD's answer on Stackoverflow :
https://stackoverflow.com/questions/16647819/timegm-cross-platform