tools: bpftool: add feature check for zlib

bpftool requires libelf, and zlib for decompressing /proc/config.gz.
zlib is a transitive dependency via libelf, and became mandatory since
elfutils 0.165 (Jan 2016). The feature check of libelf is already done
in the elfdep target of tools/lib/bpf/Makefile, pulled in by bpftool via
a dependency on libbpf.a. Add a similar feature check for zlib.
Suggested-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: NPeter Wu <peter@lekensteyn.nl>
Acked-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

tools/bpf/bpftool/Makefile

@@ -58,8 +58,8 @@ INSTALL ?= install
 RM ?= rm -f
 
 FEATURE_USER = .bpftool
-FEATURE_TESTS = libbfd disassembler-four-args reallocarray
-FEATURE_DISPLAY = libbfd disassembler-four-args
+FEATURE_TESTS = libbfd disassembler-four-args reallocarray zlib
+FEATURE_DISPLAY = libbfd disassembler-four-args zlib
 
 check_feat := 1
 NON_CHECK_FEAT_TARGETS := clean uninstall doc doc-clean doc-install doc-uninstall
@@ -111,6 +111,8 @@ OBJS = $(patsubst %.c,$(OUTPUT)%.o,$(SRCS)) $(OUTPUT)disasm.o
 $(OUTPUT)disasm.o: $(srctree)/kernel/bpf/disasm.c
 	$(QUIET_CC)$(COMPILE.c) -MMD -o $@ $<
 
+$(OUTPUT)feature.o: | zdep
+
 $(OUTPUT)bpftool: $(OBJS) $(LIBBPF)
 	$(QUIET_LINK)$(CC) $(CFLAGS) $(LDFLAGS) -o $@ $^ $(LIBS)
 
@@ -149,6 +151,9 @@ doc-uninstall:
 
 FORCE:
 
-.PHONY: all FORCE clean install uninstall
+zdep:
+	@if [ "$(feature-zlib)" != "1" ]; then echo "No zlib found"; exit 1 ; fi
+
+.PHONY: all FORCE clean install uninstall zdep
 .PHONY: doc doc-clean doc-install doc-uninstall
 .DEFAULT_GOAL := all