libyang BUGFIX set feature macro for strdup() on OSX

Since OS X requires different value (600) of _XOPEN_SOURCE feature test macro for
strdup() than linux (500), better use _POSIX_C_SOURCE 200809L. It is available
since glibc 2.12 (released 2010) so it shouldn't be an issue.
diff --git a/src/in.c b/src/in.c
index f802010..e4320f9 100644
--- a/src/in.c
+++ b/src/in.c
@@ -13,6 +13,11 @@
  */
 
 #define _GNU_SOURCE
+#define _POSIX_C_SOURCE 200809L /* strdup, strndup */
+
+#ifdef __APPLE__
+#define _DARWIN_C_SOURCE /* F_GETPATH */
+#endif
 
 #include "in.h"
 #include "in_internal.h"