Fix HOST_NAME_MAX undefined issue on MacOS

Author: houjun

src/api/pdc_timing.h

@@ -1,6 +1,14 @@
 #ifndef PDC_TIMING_H
 #define PDC_TIMING_H
 
+#ifndef HOST_NAME_MAX
+#if defined(__APPLE__)
+#define HOST_NAME_MAX 255
+#else 
+#define HOST_NAME_MAX 64
+#endif /* __APPLE__ */
+#endif /* HOST_NAME_MAX */
+
 #ifdef PDC_TIMING
 
 #include <mpi.h>