I'm using Linux Mint 17 so I did change the sim makefile to linux.
grbl and grbl-sim at the same time with git clone didn't work easy... grbl-sim has one to many directories.
In the /grbl/config.h I had to remove : #define config_h
otherwise it was not finding the default defines?
/grbl/print.c:209
I had to comment line 209 in print.c (some memory debug function, not used in sim)
/grbl/sim/simulator.c:199: renamed get_direction_mask to get_direction_pin_mask
Add the serial in front: rxbuffer* to serial_rxbuffer*
/grbl/sim/serial.c:66: extern volatile uint8_t serial_rx_buffer_head;
/grbl/sim/serial.c:67 extern volatile uint8_t serial_rx_buffer_tail;
/grbl/sim/serial.c:69: rx_buffer_head' to serial_rx_buffer_head /grbl/sim/serial.c:71:rx_buffer_tail' to serial_rx_buffer_tail
rx_buffer_head' to serial_rx_buffer_head /grbl/sim/serial.c:71:
rx_buffer_tail' to serial_rx_buffer_tail