better version information

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

Hello pari-dev,

dft.config.in should contain the PARI/GP version in simple
form, say as
version="2.2.3"

Also #include<pari.h>
should define a symbol with the pari version.

Also make install should install dft.config.in is /usr/lib/pari
(not in /usr/share since it is not shareable), so that gp2c
does not need a configured tree.

Cheers,
Bill

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