Better error message if attempting to run opengl program with version of cx without opengl compiled

[SkycoinSynth]

When we run opengl program with a version of CX without opengl support, we get "matrix not found in package gl" and weird error messages.

1> Have variable for whether OpenGL support was compiled in to CX

2> Print error message, that CX was compiled without opengl support, if program tries to import package "gl" with version of CX without opengl support.

3> If error occurs, make sure there is link to cx installation instructions