The $(LIBBPF) target should only be a dependency of prepare if the
static version of libbpf is needed. Add a new LIBBPF_STATIC variable
that is set by Makefile.config. Use LIBBPF_STATIC to determine whether
the CFLAGS, etc. need updating and for adding $(LIBBPF) as a prepare
dependency.

As Makefile.config isn't loaded for "clean" as a target, always set
LIBBPF_OUTPUT regardless of whether it is needed for $(LIBBPF). This
is done to minimize conditional logic for $(LIBBPF)-clean.

This issue and an original fix was reported by Mike Leach in:
https://lore.kernel.org/lkml/20230105172243.7238-1-mike.leach@linaro.org/

Fixes: 746bd29e ("perf build: Use tools/lib headers from install path")
Reported-by: Mike Leach <mike.leach@linaro.org>
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: bpf@vger.kernel.org
Cc: Ian Rogers <irogers@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20230106151320.619514-2-irogers@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>