--- src/systemd/sd-event.h
+++ src/systemd/sd-event.h
@@ -71,7 +71,7 @@
 typedef int (*sd_event_io_handler_t)(sd_event_source *s, int fd, uint32_t revents, void *userdata);
 typedef int (*sd_event_time_handler_t)(sd_event_source *s, uint64_t usec, void *userdata);
 typedef int (*sd_event_signal_handler_t)(sd_event_source *s, const struct signalfd_siginfo *si, void *userdata);
-#if defined _GNU_SOURCE || _POSIX_C_SOURCE >= 199309L
+#if defined _GNU_SOURCE || defined _POSIX_C_SOURCE && _POSIX_C_SOURCE >= 199309L
 typedef int (*sd_event_child_handler_t)(sd_event_source *s, const siginfo_t *si, void *userdata);
 #else
 typedef void* sd_event_child_handler_t;