math/mpc: Update to version 1.3.1

Update to MPC version 1.3.1 with the following changes:

• New function: mpc_agm
• New rounding modes "away from zero", indicated by the letter "A" and corresponding to MPFR_RNDA on the designated real or imaginary part.
• New experimental ball arithmetic.
• New experimental function: mpc_eta_fund
• Bug fixes: . mpc_asin for asin(z) with small |Re(z)| and tiny |Im(z)| . mpc_pow_fr: sign of zero part of result when the base has up to sign the same real and imaginary part, and the exponent is an even positive integer . mpc_fma: the returned int value was incorrect in some cases (indicating whether the rounded real/imaginary parts were smaller/equal/greater than the exact values), but the computed complex value was correct.