Parallel GP for windows, second try

[Bill Allombert <Bill.Allombert@math.u-bordeaux.fr>]

Dear PARI developers,

Following work by Steven Charlton at the Atelier,
I have made a second attempt at a standalone Windows GP binary that
support parallel GP:

<http://pari.math.u-bordeaux.fr/pub/pari/windows/snapshot/gppthread64-gitf6e1c13.exe>

This version should set nbthread to the number of CPU thread
automatically, like on linux.

gettime() now report the process time. Use getwalltime() to get the
real time (useful for benchmarking parallel programs).

Interrupting a parallel computation via control-C or alarm is not
supported yet.

I did not tested it on a real windows system.
It seems it is significantly faster than the previous attempt, though
it is still slower than the non-pthread version, even when threads are
not in use.

I like to thanks Steven for this work on this project.

Cheers,
Bill.