Update mpc to 1.2.1
http://www.multiprecision.org/mpc/
Bug fixes:
Fix an incompatibility problem with GMP 6.0 and before.
Fix an intermediate overflow in asin.
Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>
(cherry picked from commit 39fa0c0959c7fa56ad2baff7e1017eec30985251)