dev-libs/mpc: fix build failure against mpfr-4.0.0, bug #642300

dev-libs/mpc: fix build failure against mpfr-4.0.0, bug #642300

mprf-4.0.0 introduced new 'mpfr_fmma' symbol that collides
with mpc's 'mpfr_fmma' symbol.

It's a backport of upstream commit
https://scm.gforge.inria.fr/anonscm/gitweb?p=mpc/mpc.git;a=commitdiff;h=36a84f43f326de14db888ba07936cc9621c23f19
("use mpfr_fmma and mpfr_fmms if provided by mpfr")
which does the following to mitigate build failure:
- rename local symbol to 'mpc_fmma' to avoid collision
- reuse mpfr's symbol if that exists

Reported-by: Perfect Gentleman
Closes: https://bugs.gentoo.org/642300
Package-Manager: Portage-2.3.19, Repoman-2.3.6