build: make sdl2 and rototiller bin optional

Now that there's a decoupled libtil usable by alternative
frontends by vendoring rototiller, the build should support
fb-less rototiller-less configurations.

In lieu of this change glimmer's build requires sdl2 despite not
actually utilizing sdl_fb.  Now that shouldn't be necessary,
should there be neither libdrm or sdl2 present we'll only produce
libtil and no rototiller binary at all.

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