AVX support for hyperellratpoints

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

Dear developers,

With Michael Stoll, we have added AVX support to hyperellratpoints and
to Michael's orginal stand-alone ratpoints program.

To activate it, you need to allow the compiler to generate AVX code.
You can do
CFLAGS=-march=native ./Configure 
(this will detect if your computer support AVX).
You should see
Looking in C lib for some symbols...
...Found sse2.
...Found avx.

One good test is
H=hyperellratpoints(x^4+126*x^2+441,4*10^5);
On my computer it speed up from 10s to 4s.

Cheers,
Bill