G92 4-axis fix and Mac/Linux build error

1. Replaced backslash with POSIX-style slash for directory separator in include.
2. G92 T axis value missing offset in 4-axis mode in GCV
    
    When clicking on the zero-axes button (to the right of the Home
    button), a G92 command of the form
    
      G92 x0 y0 z0 a
    
    would be incorrectly emitted.  This change moves the toLower() method
    so that it is called on the QString instance of the axis name
    attribute directly rather than on the result of the append() method.