bpftool: Group libbfd defs in Makefile, only pass them if we use libbfd

Bpftool uses libbfd for disassembling JIT-ed programs. But the feature
is optional, and the tool can be compiled without libbfd support. The
Makefile sets the relevant variables accordingly. It also sets variables
related to libbfd's interface, given that it has changed over time.

Group all those libbfd-related definitions so that it's easier to
understand what we are testing for, and only use variables related to
libbfd's interface if we need libbfd in the first place.

In addition to make the Makefile clearer, grouping the definitions
related to disassembling JIT-ed programs will help support alternatives
to libbfd.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Tested-by: Niklas Söderlund <niklas.soderlund@corigine.com>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20221025150329.97371-5-quentin@isovalent.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

src/Makefile

@@ -118,13 +118,6 @@ ifeq ($(check_feat),1)
 include Makefile.feature
 endif
 
-ifeq ($(feature-disassembler-four-args), 1)
-CFLAGS += -DDISASM_FOUR_ARGS_SIGNATURE
-endif
-ifeq ($(feature-disassembler-init-styled), 1)
-    CFLAGS += -DDISASM_INIT_STYLED
-endif
-
 LIBS = $(LIBBPF) -lelf -lz
 LIBS_BOOTSTRAP = $(LIBBPF_BOOTSTRAP) -lelf -lz
 ifeq ($(feature-libcap), 1)
@@ -136,9 +129,7 @@ include $(wildcard $(OUTPUT)*.d)
 
 all: $(OUTPUT)bpftool
 
-BFD_SRCS = jit_disasm.c
-
-SRCS = $(filter-out $(BFD_SRCS),$(wildcard *.c))
+SRCS := $(wildcard *.c)
 
 ifeq ($(feature-libbfd),1)
   LIBS += -lbfd -ldl -lopcodes
@@ -148,9 +139,21 @@ else ifeq ($(feature-libbfd-liberty-z),1)
   LIBS += -lbfd -ldl -lopcodes -liberty -lz
 endif
 
+# If one of the above feature combinations is set, we support libbfd
 ifneq ($(filter -lbfd,$(LIBS)),)
-CFLAGS += -DHAVE_LIBBFD_SUPPORT
-SRCS += $(BFD_SRCS)
+  CFLAGS += -DHAVE_LIBBFD_SUPPORT
+
+  # Libbfd interface changed over time, figure out what we need
+  ifeq ($(feature-disassembler-four-args), 1)
+    CFLAGS += -DDISASM_FOUR_ARGS_SIGNATURE
+  endif
+  ifeq ($(feature-disassembler-init-styled), 1)
+    CFLAGS += -DDISASM_INIT_STYLED
+  endif
+endif
+ifeq ($(filter -DHAVE_LIBBFD_SUPPORT,$(CFLAGS)),)
+  # No support for JIT disassembly
+  SRCS := $(filter-out jit_disasm.c,$(SRCS))
 endif
 
 HOST_CFLAGS = $(subst -I$(LIBBPF_INCLUDE),-I$(LIBBPF_BOOTSTRAP_INCLUDE),\