As per bug 559500, cmake versions before 3.3.0-rc1 have a musl-related bug (missing sys/types.h include), which breaks the musl build. As of that version (3.3.1-r1 and 3.3.2 are both in portage), that issue is fixed upstream.
Arch teams, please test and stabilise dev-util/cmake-3.3.1-r1. Target KEYWORDS="alpha amd64 arm hppa ia64 ppc ppc64 sparc x86". Thanks!
amd64 stable
Stable for HPPA PPC64.
stable on ppc
stable on x86
stable on arm
alpha stable
ia64 stable
We will continue in bug #587666.