tools/bpf/runqslower: Fix override option for VMLINUX_BTF
The runqslower tool refuses to build without a file to read vmlinux BTF from. The build fails with an error message to override the location by setting the VMLINUX_BTF variable if autodetection fails. However, the Makefile doesn't actually work with that override - the error message is still emitted. Fix this by including the value of VMLINUX_BTF in the expansion, and only emitting the error message if the *result* is empty. Also permit running 'make clean' even though no VMLINUX_BTF is set. Fixes: 9c01546d ("tools/bpf: Add runqslower tool to tools/bpf") Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Acked-by: Andrii Nakryiko <andriin@fb.com> Link: https://lore.kernel.org/bpf/157952560237.1683545.17771785178857224877.stgit@toke.dk
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