Check for USE_XORG being non-empty instead of just defined

[AMDmi3]

Summary

The problem is that USE_XORG may be defined by other Uses modules, such as gl, but still empty.

For example

USES=gl
USE_GL=glut

leads to USE_XORG set via (in Uses/gl.mk):

USE_XORG+=  ${_GL_${_component}_USE_XORG}

to empty value, as _GL_glut_USE_XORG is not defined

This reproduces on graphics/devil when X11 option is off.

[bapt]

I am off my computer to really check but at quick glance it should probably be if defined and !empty

[AMDmi3]

In D22939#502604, @bapt wrote:

I am off my computer to really check but at quick glance it should probably be if defined and !empty

Why, wouldn't it be redundant?

[AMDmi3]

Ping

[AMDmi3]

@bapt ping

[bapt]

back on it almost 3 years later... sorry about that, after rereading, I think your proposal is ok, it would require at least an exp-run, but looks good to me.
Note I haven't tested it!!!