tools/runqslower: Enable out-of-tree build

Enable out-of-tree build for runqslower. Only set OUTPUT=.output if it
wasn't already set by the user.
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201110164310.2600671-6-jean-philippe@linaro.org

tools/bpf/runqslower/Makefile

 # SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
 include ../../scripts/Makefile.include
 
-OUTPUT := .output
+OUTPUT ?= $(abspath .output)/
+
 CLANG ?= clang
 LLC ?= llc
 LLVM_STRIP ?= llvm-strip
-DEFAULT_BPFTOOL := $(OUTPUT)/sbin/bpftool
+BPFTOOL_OUTPUT := $(OUTPUT)bpftool/
+DEFAULT_BPFTOOL := $(BPFTOOL_OUTPUT)bpftool
 BPFTOOL ?= $(DEFAULT_BPFTOOL)
 LIBBPF_SRC := $(abspath ../../lib/bpf)
-BPFOBJ := $(OUTPUT)/libbpf.a
-BPF_INCLUDE := $(OUTPUT)
+BPFOBJ_OUTPUT := $(OUTPUT)libbpf/
+BPFOBJ := $(BPFOBJ_OUTPUT)libbpf.a
+BPF_INCLUDE := $(BPFOBJ_OUTPUT)
 INCLUDES := -I$(OUTPUT) -I$(BPF_INCLUDE) -I$(abspath ../../lib)        \
        -I$(abspath ../../include/uapi)
 CFLAGS := -g -Wall
@@ -20,7 +23,6 @@ VMLINUX_BTF_PATHS := /sys/kernel/btf/vmlinux /boot/vmlinux-$(KERNEL_REL)
 VMLINUX_BTF_PATH := $(or $(VMLINUX_BTF),$(firstword			       \
 					  $(wildcard $(VMLINUX_BTF_PATHS))))
 
-abs_out := $(abspath $(OUTPUT))
 ifeq ($(V),1)
 Q =
 else
@@ -38,7 +40,11 @@ runqslower: $(OUTPUT)/runqslower
 
 clean:
 	$(call QUIET_CLEAN, runqslower)
-	$(Q)rm -rf $(OUTPUT) runqslower
+	$(Q)$(RM) -r $(BPFOBJ_OUTPUT) $(BPFTOOL_OUTPUT)
+	$(Q)$(RM) $(OUTPUT)*.o $(OUTPUT)*.d
+	$(Q)$(RM) $(OUTPUT)*.skel.h $(OUTPUT)vmlinux.h
+	$(Q)$(RM) $(OUTPUT)runqslower
+	$(Q)$(RM) -r .output
 
 $(OUTPUT)/runqslower: $(OUTPUT)/runqslower.o $(BPFOBJ)
 	$(QUIET_LINK)$(CC) $(CFLAGS) $^ -lelf -lz -o $@
@@ -59,8 +65,8 @@ $(OUTPUT)/%.bpf.o: %.bpf.c $(BPFOBJ) | $(OUTPUT)
 $(OUTPUT)/%.o: %.c | $(OUTPUT)
 	$(QUIET_CC)$(CC) $(CFLAGS) $(INCLUDES) -c $(filter %.c,$^) -o $@
 
-$(OUTPUT):
-	$(QUIET_MKDIR)mkdir -p $(OUTPUT)
+$(OUTPUT) $(BPFOBJ_OUTPUT) $(BPFTOOL_OUTPUT):
+	$(QUIET_MKDIR)mkdir -p $@
 
 $(OUTPUT)/vmlinux.h: $(VMLINUX_BTF_PATH) | $(OUTPUT) $(BPFTOOL)
 	$(Q)if [ ! -e "$(VMLINUX_BTF_PATH)" ] ; then \
@@ -70,10 +76,8 @@ $(OUTPUT)/vmlinux.h: $(VMLINUX_BTF_PATH) | $(OUTPUT) $(BPFTOOL)
 	fi
 	$(QUIET_GEN)$(BPFTOOL) btf dump file $(VMLINUX_BTF_PATH) format c > $@
 
-$(BPFOBJ): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile) | $(OUTPUT)
-	$(Q)$(MAKE) $(submake_extras) -C $(LIBBPF_SRC)			       \
-		    OUTPUT=$(abspath $(dir $@))/ $(abspath $@)
+$(BPFOBJ): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile) | $(BPFOBJ_OUTPUT)
+	$(Q)$(MAKE) $(submake_extras) -C $(LIBBPF_SRC) OUTPUT=$(BPFOBJ_OUTPUT) $@
 
-$(DEFAULT_BPFTOOL):
-	$(Q)$(MAKE) $(submake_extras) -C ../bpftool			      \
-		    prefix= OUTPUT=$(abs_out)/ DESTDIR=$(abs_out) install
+$(DEFAULT_BPFTOOL): | $(BPFTOOL_OUTPUT)
+	$(Q)$(MAKE) $(submake_extras) -C ../bpftool OUTPUT=$(BPFTOOL_OUTPUT)