Dubious removal of setgroups from setuid.c

[fweimer-rh]

#ifdef _BSD_SOURCE (added in 78799a0) is never true during the build because _BSD_SOURCE is not defined.

The root cause for the implicit declaration of setgroups is in meson.build, where c_std=c11 is specified. This results in building with -std=c11, which turns on strict POSIX mode in glibc, so setgroups is no longer declared in <grp.h>.

You probably should not downgrade the C compiler defaults using -std=c11, or define _DEFAULT_SOURCE (and perhaps _BSD_SOURCE for really old glibc) at the start of setuid.c.

[serebit]

I don't think we care about really old glibc, so we can probably ditch _BSD_SOURCE.