Backport patch from 1.7.5.

https://github.com/phaag/nfdump/commit/925dd28f7e08e00479a2906d0343e6fa75a6a783

--- a/src/lib/daemon.c
+++ b/src/lib/daemon.c
@@ -41,6 +41,7 @@
 #include <sys/types.h>
 // some linux are picky
 #define __USE_GNU
+#define _GNU_SOURCE
 #include <unistd.h>
 
 #include "util.h"