Version 8c has the arc commands commented out (g-code G2 and G3).
I use grbl version 0.9j in the Arduino and the following Post Processor files for Vcarve. They are similar to the Inventables post processors, but have the T1M6 commands removed and added an M5 command.
X_Carve_inch.pp (3.9 KB)
X_Carve_mm.pp (3.9 KB)