configure.ac: check if "struct ucred" is available

configure.ac

@@ -308,6 +308,7 @@ fi
 
 if test x$enable_un = xyes; then
 	AC_DEFINE(HAVE_UN, 1, [Define if unix domain socket support is enabled])
+	STRUCT_UCRED
 fi
 
 enable_osx=no

m4/ucred.m4

+# Check if "struct ucred" is available.  If not, try harder with
+# _GNU_SOURCE.
+#
+# Author: Max Kellermann <max@duempel.org>
+
+AC_DEFUN([STRUCT_UCRED],[
+	AC_MSG_CHECKING([for struct ucred])
+	AC_CACHE_VAL(mpd_cv_have_struct_ucred, [
+		AC_TRY_COMPILE([#include <sys/socket.h>],
+			[struct ucred cred;],
+			mpd_cv_have_struct_ucred=yes,
+			mpd_cv_have_struct_ucred=no)
+		if test x$mpd_cv_have_struct_ucred = xno; then
+			# glibc 2.8 forces _GNU_SOURCE on us
+			AC_TRY_COMPILE(
+				[#define _GNU_SOURCE
+				 #include <sys/socket.h>],
+				[struct ucred cred;],
+				mpd_cv_have_struct_ucred=yes,
+				mpd_cv_have_struct_ucred=no)
+			if test x$mpd_cv_have_struct_ucred = xyes; then
+				# :(
+				MPD_CFLAGS="$MPD_CFLAGS -D_GNU_SOURCE"
+			fi
+		fi
+		])
+
+	AC_MSG_RESULT($mpd_cv_have_struct_ucred)
+	if test x$mpd_cv_have_struct_ucred = xyes; then
+		AC_DEFINE(HAVE_STRUCT_UCRED, 1, [Define if struct ucred is present from sys/socket.h])
+	fi
+])

src/listen.c

@@ -97,7 +97,7 @@ static int establishListen(int pf, const struct sockaddr *addrp,
 	if (listen(sock, 5) < 0)
 		FATAL("problems listen'ing: %s\n", strerror(errno));
 
-#if defined(HAVE_UN) && defined(SO_PASSCRED)
+#ifdef HAVE_UCRED
 	setsockopt(sock, SOL_SOCKET, SO_PASSCRED, &passcred, sizeof(passcred));
 #endif
 
@@ -265,7 +265,7 @@ void freeAllListenSockets(void)
 
 static int get_remote_uid(int fd)
 {
-#if defined(HAVE_UN) && defined(SO_PEERCRED)
+#ifdef HAVE_UCRED
 	struct ucred cred;
 	socklen_t len = sizeof (cred);