Issue #594 - provide an OVERRIDE_GET_RANDOM_SEED cmake variable to override json_c_get_random_seed() for embedded platforms where time(NULL) doesn't work.

Example:
mkdir build && cd build
cmake -DOVERRIDE_GET_RANDOM_SEED='do { extern uint32_t getMsTicks(void); int ms = getMsTicks() * 433494437; return ms; } while(0)' ..

random_seed.c

@@ -276,6 +276,9 @@ static int get_time_seed(void)
 
 int json_c_get_random_seed(void)
 {
+#ifdef OVERRIDE_GET_RANDOM_SEED
+	OVERRIDE_GET_RANDOM_SEED;
+#endif
 #if defined HAVE_RDRAND && HAVE_RDRAND
 	if (has_rdrand())
 		return get_rdrand_seed();