[gtkada] Configure doesn't find openGL

[Emmanuel Briot]

Pascal, Roger,

There is now and then demand for openGL with GtkAda. Would you guys be interested
in providing a pull request (on github) for GtkAda so that we can fix/include such support ?
Seems you are pretty close to having everything working correctly.

Such support is unlikely to come from us directly, since there is no commercial demand for
it, but of course we are interested in having GtkAda be as complete/good as possible...

Thanks in advance

Emmanuel