signal, spawn: use link warnings These modules depended on link-warning, then didn't use it. * lib/signal.in.h (sigset_t): Make unconditional. (sigismember, sigemptyset, sigaddset, sigdelset, sigfillset) (sigpending, sigprocmask, sigaction): Add link warnings. * lib/spawn.in.h (posix_spawn, posix_spawnp, posix_spawnattr_init) (posix_spawnattr_destroy, posix_spawnattr_getsigdefault) (posix_spawnattr_setsigdefault, posix_spawnattr_getsigmask) (posix_spawnattr_setsigmask, posix_spawnattr_getflags) (posix_spawnattr_setflags, posix_spawnattr_getpgroup) (posix_spawnattr_setpgroup, posix_spawnattr_getschedpolicy) (posix_spawnattr_setschedpolicy, posix_spawnattr_getschedparam) (posix_spawnattr_setschedparam, posix_spawn_file_actions_init) (posix_spawn_file_actions_destroy) (posix_spawn_file_actions_addopen) (posix_spawn_file_actions_addclose) (posix_spawwn_file_actions_adddup2): Likewise. * m4/signal_h.m4 (gl_SIGNAL_H): Guarantee uid_t. * tests/test-signal.c (main): Enhance test. Signed-off-by: Eric Blake <ebb9@byu.net>