meson/android/scons/gallium: always define LLVM_AVAILABLE one way or the other

With the usual benefits of `#if` instead of `#ifdef`; mostly the fact
that typos are now build failures instead of silently being ignored.

Signed-off-by: Eric Engestrom <>
41 jobs for !3863 with llvm_available-always-defined in 17 minutes and 2 seconds (queued for 7 seconds)
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