selftests/bpf: Enforce libbpf build before BPF programs are built

Given BPF programs rely on libbpf's bpf_helper_defs.h, which is
auto-generated during libbpf build, libbpf build has to happen before
we attempt progs/*.c build. Enforce it as order-only dependency.

Fixes: 24f25763 ("libbpf: auto-generate list of BPF helper definitions")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191011220146.3798961-2-andriin@fb.com

tools/testing/selftests/bpf/Makefile

@@ -256,7 +256,8 @@ ifeq ($(DWARF2BTF),y)
 	$(BTF_PAHOLE) -J $@
 endif
 
-$(OUTPUT)/%.o: progs/%.c
+# libbpf has to be built before BPF programs due to bpf_helper_defs.h
+$(OUTPUT)/%.o: progs/%.c | $(BPFOBJ)
 	($(CLANG) $(BPF_CFLAGS) $(CLANG_CFLAGS) -O2 -target bpf -emit-llvm \
 		-c $< -o - || echo "clang failed") | \
 	$(LLC) -march=bpf -mcpu=$(CPU) $(LLC_FLAGS) -filetype=obj -o $@