configure.ac, unix/Daemon: check for initgroups() at configure time

The initgroups() manpage says we need to check for _BSD_SOURCE.  The
thing is that glibc deprecated this macro, and doesn't define it
anymore, effectively breaking all MPD supplementary groups.

The real fix is to check for initgroups() availability at configure
time, instead of relying on the deprecated _BSD_SOURCE macro.

NEWS

@@ -3,6 +3,7 @@ ver 0.19.13 (not yet released)
   - aiff, riff: fix ID3 chunk padding
 * decoder
   - ffmpeg: support the TAK codec
+* initialize supplementary groups with glibc 2.19+
 
 ver 0.19.12 (2015/12/15)
 * fix assertion failure on malformed UTF-8 tag

configure.ac

@@ -206,6 +206,7 @@ if test x$host_is_linux = xyes; then
 fi
 
 AC_CHECK_FUNCS(getpwnam_r getpwuid_r)
+AC_CHECK_FUNCS(initgroups)
 AC_CHECK_FUNCS(strndup)
 
 if test x$host_is_linux = xyes; then

src/unix/Daemon.cxx

@@ -110,7 +110,7 @@ daemonize_set_user(void)
 				       (int)user_gid);
 	}
 
-#ifdef _BSD_SOURCE
+#ifdef HAVE_INITGROUPS
 	/* init supplementary groups
 	 * (must be done before we change our uid)
 	 */