Comment # 1 on bug 109714 from Adam Jackson

(In reply to moriel5 from comment #0)

> When attempting to compile Mesa with the R200 driver, on Solus, Meson spits
> out the following error: ERROR:  Options "r200" are not in allowed choices:
> ", auto, kmsro, radeonsi, r300, r600, nouveau, freedreno, swrast, v3d, vc4,
> etnaviv, tegra, i915, svga, virgl, swr, panfrost".

That's the list of gallium drivers. r200 is not a gallium driver, it's a
"classic" DRI driver:

datura:~/git/mesa% meson configure build | grep r200
  dri-drivers        [auto]        [, auto, i915, i965, r100, r200, nouveau,
swrast]                                                                        
    List of dri drivers to build. If this is set to auto all drivers applicable
to the target OS/architecture will be built  

You want 'meson configure build -Ddri-drivers=r200'.