Merge from vendor branch TNF:

Merge from vendor branch TNF:
Import MPC (multiprecision complex arithmetic library) version 0.8.2
as math/mpcomplex.
Packaged by Marko Schuetz for pkgsrc-wip.

MPC is a C library for the arithmetic of complex numbers with
arbitrarily high precision and correct rounding of the result.
It is built upon and follows the same principles as MPFR.