MINOR: tools: add a portable timegm() alternative

MINOR: tools: add a portable timegm() alternative

timegm() is not provided everywhere and the documentation on how to
replace it is bogus as it proposes an inefficient and non-thread safe
alternative.

Here we reimplement everything needed to compute the number of seconds
since Epoch based on the broken down fields in struct tm. It is only
guaranteed to return correct values for correct inputs. It was successfully
tested with all possible 32-bit values of time_t converted to struct tm
using gmtime() and back to time_t using the legacy timegm() and this
function, and both functions always produced the same result.

Thanks to Benoît Garnier for an instructive discussion and detailed
explanations of the various time functions, leading to this solution.