Add a glaze_execute function to libglaze

This function takes care of all of the magic needed in order to use Glaze.

With this function, the caller need not set the environment variables
such as LD_LIBRARY_PATH, GLAZE_WRAPPER, and GLAZE_LIBGL that would
otherwise need to be set in order to use Glaze.

In some cases, (such as when the "program" passed to glaze_execute is
a script that forces an alternate libGL.so.1 to be loaded), the user
may still need to set GLAZE_LIBGL to the absolute path of the correct,
underlying libGL.so.1 providing the implementation of OpenGL.