Merge pull request #1 from srichy/Letartare

G92 4-axis fix and Mac/Linux build error
Linux bld fails with libQGLViewer2 and file case

According to http://www.libqglviewer.com/compilation.html the correct
spec for linking to libQGLViewer on Windows shows the trailing '2', but
not for Linux.  Also, the Linux build requires -lGLU (I presume that
the GL Utility library is bundled with the normal OpenGL libs on
Windows and Mac).