bpftool: Do not FORCE-build libbpf

In bpftool's Makefile, libbpf has a FORCE dependency, to make sure we
rebuild it in case its source files changed. Let's instead make the
rebuild depend on the source files directly, through a call to the
"$(wildcard ...)" function. This avoids descending into libbpf's
directory if there is nothing to update.

Do the same for the bootstrap libbpf version.

This results in a slightly faster operation and less verbose output when
running make a second time in bpftool's directory.

Before:

    Auto-detecting system features:
    ...                        libbfd: [ on  ]
    ...        disassembler-four-args: [ on  ]
    ...                          zlib: [ on  ]
    ...                        libcap: [ on  ]
    ...               clang-bpf-co-re: [ on  ]

    make[1]: Entering directory '/root/dev/linux/tools/lib/bpf'
    make[1]: Entering directory '/root/dev/linux/tools/lib/bpf'
    make[1]: Nothing to be done for 'install_headers'.
    make[1]: Leaving directory '/root/dev/linux/tools/lib/bpf'
    make[1]: Leaving directory '/root/dev/linux/tools/lib/bpf'

After:

    Auto-detecting system features:
    ...                        libbfd: [ on  ]
    ...        disassembler-four-args: [ on  ]
    ...                          zlib: [ on  ]
    ...                        libcap: [ on  ]
    ...               clang-bpf-co-re: [ on  ]

Other ways to clean up the output could be to pass the "-s" option, or
to redirect the output to >/dev/null, when calling make recursively to
descend into libbpf's directory. However, this would suppress some
useful output if something goes wrong during the build. A better
alternative would be to pass "--no-print-directory" to the recursive
make, but that would still leave us with some noise for
"install_headers". Skipping the descent into libbpf's directory if no
source file has changed works best, and seems the most logical option
overall.
Reported-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20211009210341.6291-3-quentin@isovalent.com

tools/bpf/bpftool/Makefile

@@ -42,7 +42,7 @@ endif
 $(LIBBPF_OUTPUT) $(BOOTSTRAP_OUTPUT) $(LIBBPF_BOOTSTRAP_OUTPUT):
 	$(QUIET_MKDIR)mkdir -p $@
 
-$(LIBBPF): FORCE | $(LIBBPF_OUTPUT)
+$(LIBBPF): $(wildcard $(BPF_DIR)/*.[ch] $(BPF_DIR)/Makefile) | $(LIBBPF_OUTPUT)
 	$(Q)$(MAKE) -C $(BPF_DIR) OUTPUT=$(LIBBPF_OUTPUT) \
 		DESTDIR=$(LIBBPF_DESTDIR) prefix= $(LIBBPF) install_headers
 
@@ -50,7 +50,7 @@ $(LIBBPF_INTERNAL_HDRS): $(LIBBPF_HDRS_DIR)/%.h: $(BPF_DIR)/%.h $(LIBBPF)
 	$(call QUIET_INSTALL, $@)
 	$(Q)install -m 644 -t $(LIBBPF_HDRS_DIR) $<
 
-$(LIBBPF_BOOTSTRAP): FORCE | $(LIBBPF_BOOTSTRAP_OUTPUT)
+$(LIBBPF_BOOTSTRAP): $(wildcard $(BPF_DIR)/*.[ch] $(BPF_DIR)/Makefile) | $(LIBBPF_BOOTSTRAP_OUTPUT)
 	$(Q)$(MAKE) -C $(BPF_DIR) OUTPUT=$(LIBBPF_BOOTSTRAP_OUTPUT) \
 		ARCH= CC=$(HOSTCC) LD=$(HOSTLD) $@