meson: libelf is optional
..otherwise seeing ERROR: C shared or static library 'elf' not found
at least on Android. I think the intention of !18503 (merged) is to make it optional? Over to @lygstate to confirm.
Fixes: c74595ea ("radv/r600/clover: Getting libelf to be optional")
/cc @lygstate
Edited by Yiwei Zhang