tools/bpf: Don't use $(COMPILE.c)

When using make kselftest TARGETS=bpf, tools/bpf is built with
MAKEFLAGS=rR, which causes $(COMPILE.c) to be undefined, which in turn
causes the build to fail with

  CC       kselftest/bpf/tools/build/bpftool/map_perf_ring.o
/bin/sh: 1: -MMD: not found

Fix by using $(CC) $(CFLAGS) -c instead of $(COMPILE.c).
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200602175649.2501580-2-iii@linux.ibm.com

tools/bpf/Makefile

@@ -64,12 +64,12 @@ $(OUTPUT)%.lex.c: $(srctree)/tools/bpf/%.l
 	$(QUIET_FLEX)$(LEX) -o $@ $<
 
 $(OUTPUT)%.o: $(srctree)/tools/bpf/%.c
-	$(QUIET_CC)$(COMPILE.c) -o $@ $<
+	$(QUIET_CC)$(CC) $(CFLAGS) -c -o $@ $<
 
 $(OUTPUT)%.yacc.o: $(OUTPUT)%.yacc.c
-	$(QUIET_CC)$(COMPILE.c) -o $@ $<
+	$(QUIET_CC)$(CC) $(CFLAGS) -c -o $@ $<
 $(OUTPUT)%.lex.o: $(OUTPUT)%.lex.c
-	$(QUIET_CC)$(COMPILE.c) -o $@ $<
+	$(QUIET_CC)$(CC) $(CFLAGS) -c -o $@ $<
 
 PROGS = $(OUTPUT)bpf_jit_disasm $(OUTPUT)bpf_dbg $(OUTPUT)bpf_asm
 

tools/bpf/bpftool/Makefile

@@ -126,7 +126,7 @@ else
 endif
 
 $(OUTPUT)_prog.o: prog.c
-	$(QUIET_CC)$(COMPILE.c) -MMD -DBPFTOOL_WITHOUT_SKELETONS -o $@ $<
+	$(QUIET_CC)$(CC) $(CFLAGS) -c -MMD -DBPFTOOL_WITHOUT_SKELETONS -o $@ $<
 
 $(OUTPUT)_bpftool: $(_OBJS) $(LIBBPF)
 	$(QUIET_LINK)$(CC) $(CFLAGS) $(LDFLAGS) -o $@ $(_OBJS) $(LIBS)
@@ -141,10 +141,10 @@ profiler.skel.h: $(OUTPUT)_bpftool skeleton/profiler.bpf.o
 	$(QUIET_GEN)$(OUTPUT)./_bpftool gen skeleton skeleton/profiler.bpf.o > $@
 
 $(OUTPUT)prog.o: prog.c profiler.skel.h
-	$(QUIET_CC)$(COMPILE.c) -MMD -o $@ $<
+	$(QUIET_CC)$(CC) $(CFLAGS) -c -MMD -o $@ $<
 
 $(OUTPUT)disasm.o: $(srctree)/kernel/bpf/disasm.c
-	$(QUIET_CC)$(COMPILE.c) -MMD -o $@ $<
+	$(QUIET_CC)$(CC) $(CFLAGS) -c -MMD -o $@ $<
 
 $(OUTPUT)feature.o: | zdep
 
@@ -152,7 +152,7 @@ $(OUTPUT)bpftool: $(__OBJS) $(LIBBPF)
 	$(QUIET_LINK)$(CC) $(CFLAGS) $(LDFLAGS) -o $@ $(__OBJS) $(LIBS)
 
 $(OUTPUT)%.o: %.c
-	$(QUIET_CC)$(COMPILE.c) -MMD -o $@ $<
+	$(QUIET_CC)$(CC) $(CFLAGS) -c -MMD -o $@ $<
 
 clean: $(LIBBPF)-clean
 	$(call QUIET_CLEAN, bpftool)