Update GMP/MPFR build instructions for SPARC/Solaris

[Eric Botcazou]

> Oh, that.  Yes, I use sun's cc with gmp 4.2.1 on solaris 10.  I solved the
> problem from the PR by setting ABI=32 as per the instructions here:
> http://www.swox.com/gmp/manual/ABI-and-ISA.html#ABI-and-ISA

Not a small problem in my opinion. :-)

> I'm not sure whether using --build= pessimizes gmp to use generic sparc
> instructions as opposed to ultrasparc.  So maybe recommending ABI=32 is
> better?

I think the safest stance is to use --build=, it's more standard stuff.

-- 
Eric Botcazou