Define _POSIX_C_SOURCE only if it is not defined

Fixes https://github.com/behdad/harfbuzz/pull/45
diff --git a/src/hb-blob.cc b/src/hb-blob.cc
index d6acca0..b82b4b2 100644
--- a/src/hb-blob.cc
+++ b/src/hb-blob.cc
@@ -25,7 +25,9 @@
  */
 
 /* http://www.oracle.com/technetwork/articles/servers-storage-dev/standardheaderfiles-453865.html */
+#ifndef _POSIX_C_SOURCE
 #define _POSIX_C_SOURCE 199309L
+#endif
 
 #include "hb-private.hh"