Compiling dev-libs/mpfr-3.1.4 on Cygwin requires this upstream patch: https://gforge.inria.fr/scm/viewvc.php/mpfr/trunk/src/mpfr-impl.h?r1=10257&r2=10260 Thanks!
added here: https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=9f00d603354fbbb958798a6d8652b8fd2cece88d