Support Library Requirements for GCC 7.1

[Joel Sherrill <joel dot sherrill at oarcorp dot com>]

Hi

I am trying to update the gcc version for rtems to 7.1 and
running into trouble finding the correct versions of
mpc, mpfr, and gmp. We build those as part of building
gcc so we have configuration control over the set.

With gcc 6.3.0, we have this in our build recipe:

%define mpfr_version   2.4.2
%define mpc_version    0.8.1
%define gmp_version    4.3.2

I tried that with gcc 7.1.0 but the build failed complaining
mpfr was too old. I upgraded mpfr and them got a
compiler error which appeared to be GMP related so I bumped
GMP. The same error remained:

../../../gcc-7.1.0/mpc/src/acos.c: In function 'mpc_acos':
../../../gcc-7.1.0/mpc/src/acos.c:192:19: error: 'GMP_RNDA' undeclared (first use in this function)
       : rnd_im == GMP_RNDA ? GMP_RNDZ

I bumped MPC to 1.0.3 which worked. Now we are using this
in the build recipe.

%define mpfr_version   3.1.5
%define mpc_version    1.0.3
%define gmp_version    6.1.2

The gcc-7.1.0/INSTALL/prerequisites.html seem to be
out of date. I am sure the minimum is lower than what
I ended up using.

What are the correct minimums for gcc 7.1?

Thanks.

-joel