https://bugs.freedesktop.org/show_bug.cgi?id=100393
--- Comment #1 from Ilia Mirkin imirkin@alum.mit.edu --- You're forcing a core context, which in turn disables things like glGetString(GL_EXTENSIONS); you have to use glGetStringi now. However cemu isn't ready for a core context.
Try MESA_GL_VERSION_OVERRIDE=4.0COMPAT -- that should have the desired effect. Note that tons of stuff doesn't work in such high-versioned compat contexts. Any time you're setting any overrides, it's your problem rather than a mesa problem...