Fix build on musl libc by exposing setgroups(2).

setgroups(2) is a BSD syscall. GNU libc exposes it by default but musl
libc requires a feature flag to get that syscall defined.

The most compatible single flag is _BSD_SOURCE (which musl and GNU libc
both respect, and which FreeBSD ignores without going into their own
version of strict mode). However recent glibc requires _DEFAULT_SOURCE
instead, which no other libc seems to care about. So I defined both to
keep everyone happy...

Differential Revision: https://phabricator.kde.org/D6597