bpftool: Remove zlib feature test from Makefile

The feature test to detect the availability of zlib in bpftool's
Makefile does not bring much. The library is not optional: it may or may
not be required along libbfd for disassembling instructions, but in any
case it is necessary to build feature.o or even libbpf, on which bpftool
depends.

If we remove the feature test, we lose the nicely formatted error
message, but we get a compiler error about "zlib.h: No such file or
directory", which is equally informative. Let's get rid of the test.
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220705200456.285943-1-quentin@isovalent.com

tools/bpf/bpftool/Makefile

@@ -93,10 +93,8 @@ INSTALL ?= install
 RM ?= rm -f
 
 FEATURE_USER = .bpftool
-FEATURE_TESTS = libbfd disassembler-four-args zlib libcap \
-	clang-bpf-co-re
-FEATURE_DISPLAY = libbfd disassembler-four-args zlib libcap \
-	clang-bpf-co-re
+FEATURE_TESTS = libbfd disassembler-four-args libcap clang-bpf-co-re
+FEATURE_DISPLAY = libbfd disassembler-four-args libcap clang-bpf-co-re
 
 check_feat := 1
 NON_CHECK_FEAT_TARGETS := clean uninstall doc doc-clean doc-install doc-uninstall
@@ -204,11 +202,6 @@ $(BOOTSTRAP_OUTPUT)disasm.o: $(srctree)/kernel/bpf/disasm.c
 $(OUTPUT)disasm.o: $(srctree)/kernel/bpf/disasm.c
 	$(QUIET_CC)$(CC) $(CFLAGS) -c -MMD $< -o $@
 
-$(OUTPUT)feature.o:
-ifneq ($(feature-zlib), 1)
-	$(error "No zlib found")
-endif
-
 $(BPFTOOL_BOOTSTRAP): $(BOOTSTRAP_OBJS) $(LIBBPF_BOOTSTRAP)
 	$(QUIET_LINK)$(HOSTCC) $(HOST_CFLAGS) $(LDFLAGS) $(BOOTSTRAP_OBJS) $(LIBS_BOOTSTRAP) -o $@