complibs/mpc: fix MPC 0.8.1 build with MPFR 3.0.0

This is the change introduced by revision 734 of MPC repository.

Author: Paul Zimmermann <Paul.Zimmermann@loria.fr>
Revision log: [acos.c] fixed problem with GMP_RNDA (should be MPFR_RNDA, and code was wrong)

Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>

 # m68k specific configuration file

 # depends on EXPERIMENTAL

 config ARCH_m68k

     select ARCH_SUPPORTS_32

     select ARCH_DEFAULT_32

     select ARCH_DEFAULT_BE

     select ARCH_SUPPORT_CPU

      help

       The m68k architecture