Fix sim makefile so it works on mac

These changes include a path separator fix and the removal of --gc-sections which causes ld failures, and is not needed on a pc.

This patch also changes how a compiler is selected.  The makefile will now select the system compiler , which should work fine
under mingw and linux.
Merge pull request #167 from tmpvar/simulator-mac

Fix sim makefile so it works on mac
Merge bug fixes from recent v0.8c push. Added readme for Grbl Sim.
Pushed minor changes. Thanks @Protoneer!
G-code updates for G10 L2 and L20.

- LinuxCNC's g-code definitions changed for G10. Updated to their
descriptions.
Untested! Soft limits, max travel, homing changes, new settings.

- WARNING: Completely untested. Will later when there is time. Settings
WILL be overwritten, as there are new settings.

- Soft limits installed. Homing must be enabled for soft limits to work
correctly. Errors out much like a hard limit, locking out everything
and bringing up the alarm mode. Only difference is it forces a feed
hold before doing so. Position is not lost.

- IMPORTANT: Homing had to be updated so that soft limits work better
with less CPU overhead. When homing completes, all axes are assumed to
exist in negative space. If your limit switch is other side, the homing
cycle with set this axis location to the max travel value, rather than
zero.

- Update mc_line() to accept an array, rather than individual variables.

- Added an mc_auto_cycle_start() function handle this feature.
Organization only.

-
Update README.md
Minor updates to code and commenting.