Issue #594 - provide an OVERRIDE_GET_RANDOM_SEED cmake variable to override json_c_ge...

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)' ..