Update _POSIX_C_SOURCE to the latest version

Defining _POSIX_C_SOURCE to an old version on FreeBSD can cause C99 to
be disabled in libc.

src/hb-blob.cc

@@ -26,7 +26,7 @@
 
 /* http://www.oracle.com/technetwork/articles/servers-storage-dev/standardheaderfiles-453865.html */
 #ifndef _POSIX_C_SOURCE
-#define _POSIX_C_SOURCE 199309L
+#define _POSIX_C_SOURCE 200809L
 #endif
 
 #include "hb-private.hh"