always define _GNU_SOURCE for linux

We are using various extenstions that the spec say depends on _GNU_SOURCE,
for example unshare, CPU_SET, CPU_ZERO, cpu_set_t. We enable those always
for linux and we never unset it.

Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
define WAIT_ANY if missing

POSIX uses -1 and does not define WAIT_ANY so we need to define it if
needed.

See:
http://pubs.opengroup.org/onlinepubs/9699919799/functions/waitpid.html
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/sys_wait.h.html

Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>