Makefile changes

Created by: sonninnos

• retro_get_system_info() assumes GIT_VERSION always exists, therefore always include it Closes #232 (closed)
• Relocated -s away from interfering with Windows debug build
  • No harm having it globally with regular builds?
• Enabled SERIAL_PORT, which fixes:
  • All versions of Super Skidmarks, except that WHDLoad slave 1.1 with the ludicrous memory requirement
  • Grand Prix Circuit