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LIBBPF is dependent on zlib so move the NO_ZLIB and feature check
early to avoid statically building when zlib is disabled. This avoids
a linkage failure with perf and static libbpf when zlib isn't
specified.

Move BUILD_BPF_SKEL logic to one place and if not defined set
BUILD_BPF_SKEL to 1. Detect dependencies of building with BPF
skeletons and warn/disable if the dependencies aren't present.

Change Makefile.perf to contain BPF skeleton logic dependent on the
Makefile.config result and refresh the comment about BUILD_BPF_SKEL.
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Namhyung Kim <namhyung@kernel.org>
Cc: James Clark <james.clark@arm.com>
Cc: Nick Terrell <terrelln@fb.com>
Cc: Patrice Duroux <patrice.duroux@gmail.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Cc: Tiezhu Yang <yangtiezhu@loongson.cn>
Cc: Tom Rix <trix@redhat.com>
Cc: llvm@lists.linux.dev
Cc: bpf@vger.kernel.org
Link: https://lore.kernel.org/r/20230914211948.814999-3-irogers@google.com

Signed-off-by: Namhyung Kim <namhyung@kernel.org>