MINOR: systemd: replace SOCK_CLOEXEC by fcntl call to FD_CLOEXEC

Since we build systemd.o for every target, we need it to be more
portable.

The SOCK_CLOEXEC argument from socket() is not portable and won't build
on some OS like macOS X.

This patch fixes the issue by replace SOCK_CLOEXEC by a fnctl set to
FD_CLOEXEC.

Author: wlallemand

src/systemd.c

@@ -10,6 +10,7 @@
  */
 
 #include <errno.h>
+#include <fcntl.h>
 #include <inttypes.h>
 #include <signal.h>
 #include <stdbool.h>
@@ -87,12 +88,17 @@ int sd_notify(int unset_environment, const char *message)
 	if (socket_addr.sun.sun_path[0] == '@')
 		socket_addr.sun.sun_path[0] = 0;
 
-	fd = socket(AF_UNIX, SOCK_DGRAM|SOCK_CLOEXEC, 0);
+	fd = socket(AF_UNIX, SOCK_DGRAM, 0);
 	if (fd < 0) {
 		ret = -errno;
 		goto end;
 	}
 
+	if (fcntl(fd, F_SETFD, FD_CLOEXEC) != 0) {
+		ret = -errno;
+		goto end;
+	}
+
 	if (connect(fd, &socket_addr.sa, offsetof(struct sockaddr_un, sun_path) + path_length) != 0) {
 		ret = -errno;
 		goto end;