ABI chosen by GMP and documentation

[Vincent Lefevre]

On some 64-bit machines, the GMP library isn't built with the default
ABI. This seems to confuse many users. I think that this should be
documented in the INSTALL file (early enough, as most users don't
read it entirely) and (because many users probably don't read the
INSTALL file at all) at the end of "make install".

Thanks,

-- 
Vincent Lefèvre <vincent at vinc17.org> - Web: <http://www.vinc17.org/>
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Work: CR INRIA - computer arithmetic / Arenaire project (LIP, ENS-Lyon)