Lowering compiler optimisation with Apple gcc

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

Hello PARI-dev,

we regurlarly get bug reports about random problems on Apple platform,
see #504, #523,
<http://pari.math.u-bordeaux.fr/archives/pari-dev-0603/msg00003.html>
<http://pari.math.u-bordeaux.fr/archives/pari-dev-0606/msg00004.html>
and others.

Unfortunately we never have been able to reproduce any of them and
they looks like compilers bugs in various Apple gcc build.

Maybe we should lower the default optimisation level to reduce breakage.
Advanced users could still pick higher optimisation level.

We could check the gcc version for the string 'Apple'.

What do you think ?

Cheers,
Bill