When dealing with swrast, there's two possibilities: If you have LLVM, you get
llvmpipe, which is pretty fast. If you don't, you get softpipe, which is slow,
but does have a couple nice qualities, like being smaller and not needing
executable memory for JIT.
If you're building a driver that requires LLVM like radeonsi then you need the
LLVM stub for the build to find LLVM. But for swrast, since it can mean either
softpipe/llvmpipe, you don't strictly need LLVM. So this just makes the
Android build files flexible like the Meson build files (where you can specify
-Dllvm=disabled even if LLVM is findable).
Reviewed-by: Roman Stratiienko <r.stratiienko@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13532>
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