Improved Windows stand-alone GP binary

[Bill Allombert <Bill.Allombert@math.u-bordeaux1.fr>]

Dear PARI-dev,

I have improved the build process for the windows stand-alone GP binary.
Now binaries are built with readline support and with optional GMP5 support.
As always they are available at

<http://pari.math.u-bordeaux.fr/pub/pari/windows/snapshots/>
Please test.
If the binaries works correctly, we might be able to provide an installer
package again (using mingw instead of cygwin).

Known problems:
1) gp assumes that the terminal is not ANSI, so uses win32 API call to change colors
instead of ANSI color sequences (the conversion code was provided by Vasili
Burdo). In practice this means that colors work in the windows console, but not
in pure ANSI terminals.

2) as a side effect, color prompt does not work,  because the prompt is not
subject to the conversion above.

Cheers,
Bill.