Rename feature variable clang-bpf-global-var following upstream changes.
This restores the HAVE_CLANG feature override and should avoid rare build
errors where a recent host clang and BTF-enabled host kernel are present.
Fixes: 23be333401f0 ("bpftools: update to 5.10.10")
Signed-off-by: Tony Ambardar <itugrok@yahoo.com>
(cherry picked from commit cf20f1bb5f0479c2509dd651d08e235a3b9e8755)
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