Add options ENABLE_GLX_PRELOAD and GLX_LD_PRELOAD

[Djelff]

On some systems, in particular on systems with the NVidia driver on server, software glx drivers are not available.
This problem is not only with us, read this: https://www.nomachine.com/AR01Q01012

The solution is to add "export LD_PRELOAD=/some_dir/libGL.so" to /home/user/.bashrc
However, such a solution is not suitable if the user works both locally and remotely and complicates server administration.

I suggest adding options ENABLE_GLX_PRELOAD=1/0 and GLX_LD_PRELOAD=/some_dir/lib.so
In this case, it will be possible to choose the fastest library, for example LLVMpipe or VirtualGL.

[dimbor-ru]

Does nxagent need to preload this library at startup? Not userapp only? freenx-server had a similar mechanism named SET_LD_LIBRARY_PATH (Now disabled for new nx-libs). It can help us, or we need to LD_PRELOAD for app immediate?

[Djelff]

Yes, I noticed that a similar mechanism used to be.
No, this is not urgent, I work only remotely.
But worth implementing.

[uli42]

nxagent does not need that. It is the clients within the session the require it. I think the proper way is to provide a decent wrapper that takes care of that, say nxglxrun, which can be used to start an app. See https://wiki.x2go.org/doku.php/wiki:development:glx-xlib-workaround