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>

reporter_name="Yann E. MORIN"

reporter_url="http://ymorin.is-a-geek.org/projects/crosstool"

reporter_comment="Experimental toolchain for ARM cortex-m3 in Thumb-only, noMMU, and newlib."