samples/bpf: Do not FORCE-recompile libbpf

In samples/bpf/Makefile, libbpf has a FORCE dependency that force it to
be rebuilt. I read this as a way to keep the library up-to-date, given
that we do not have, in samples/bpf, a list of the source files for
libbpf itself. However, a better approach would be to use the
"$(wildcard ...)" function from make, and to have libbpf depend on all
the .c and .h files in its directory. This is what samples/bpf/Makefile
does for bpftool, and also what the BPF selftests' Makefile does for
libbpf.

Let's update the Makefile to avoid rebuilding libbpf all the time (and
bpftool on top of it).

Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211007194438.34443-11-quentin@isovalent.com

samples/bpf/Makefile

@@ -276,7 +276,7 @@ clean:
 	@find $(CURDIR) -type f -name '*~' -delete
 	@$(RM) -r $(CURDIR)/libbpf $(CURDIR)/bpftool
 
-$(LIBBPF): FORCE | $(LIBBPF_OUTPUT)
+$(LIBBPF): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile) | $(LIBBPF_OUTPUT)
 # Fix up variables inherited from Kbuild that tools/ build system won't like
 	$(MAKE) -C $(LIBBPF_SRC) RM='rm -rf' EXTRA_CFLAGS="$(TPROGS_CFLAGS)" \
 		LDFLAGS=$(TPROGS_LDFLAGS) srctree=$(BPF_SAMPLES_PATH)/../../ \