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/context.c b/src/context.c
index e345804..59c7093 100644
--- a/src/context.c
+++ b/src/context.c
@@ -12,6 +12,7 @@
  *     https://opensource.org/licenses/BSD-3-Clause
  */
 #define _GNU_SOURCE /* asprintf */
+#define _POSIX_C_SOURCE 200809L /* strdup */
 
 #include "context.h"