It's not that your OpenGL libraries are not found, it's more likely that extremetuxracer won't compile against them. Build/configure scripts aren't smart, if compile tests fail during configuration it reports the libraries as "not found".
The includes and libraries come from glvnd now, those are the front facing interfaces. If you have libglvnd installed, you have them. Mesa is the back end that glvnd dispatches to, when using it. (It would dispatch to the nvidia libraries if you have the proprietary driver installed but you'd still be compiling against the glvnd interfaces)
Do you have the latest sources, dated 2024-05-09?
https://sourceforge.net/projects/extrem ... ses/0.8.4/
P.S. I got it to configure, but make bombed out on my SFML (installed Arch distro package) audio includes immediately so I can't get far enough to see if the opengl stuff compiles and links. I'd probably have to build an older version for this. It's probably not going to be easy to compile extremetuxracer anymore.
It looks like it's going to need both older libFLAC and older SFML (at least older than I have) versions to compile it. Old versions like etr 0.4 used SDL for those interfaces, but newer sources use sfml instead. Slackware doesn't provide it.
It would be possible to compile and install acceptable versions in alternate locations and use environment variables for the build. However, I'm not sure how important this is to you, so I'm not going to go any further unless you care enough
