mruby-time: protect against incorrectly defined _POSIX_TIMERS

This update avoids the following error: operator '&&' has no right operand (related to _POSIX_TIMERS)
See: https://stackoverflow.com/questions/48538243/macro-if-statement-returns-error-operator-has-no-right-operand

mrbgems/mruby-time/src/time.c

@@ -83,6 +83,12 @@ double round(double x) {
 
 /** end of Time class configuration */
 
+/* protection against incorrectly defined _POSIX_TIMERS */
+#if (_POSIX_TIMERS + 0) == 0
+#undef _POSIX_TIMERS
+#define _POSIX_TIMERS 0
+#endif
+
 #if (defined(_POSIX_TIMERS) && _POSIX_TIMERS > 0) && defined(CLOCK_REALTIME)
 # define USE_CLOCK_GETTIME
 #endif