Guard _DEFAULT_SOURCE against redefinition

GCC with -Werror treats macro redefinition as an error.
Some compilers or build systems pre-define _DEFAULT_SOURCE.

Author: renaudallard

thinproxy.c

@@ -24,7 +24,9 @@
  * POSSIBILITY OF SUCH DAMAGE.
  */
 
+#ifndef _DEFAULT_SOURCE
 #define _DEFAULT_SOURCE
+#endif
 
 #include <sys/types.h>
 #include <sys/socket.h>