Change libbpf install_headers DESTDIR to "build" and cleanup

The libbpf headers got installed into ../libbpf/src/root via
invoking make install_headers with DESTDIR=root.

This patch change the clean target to also cleanup this DESTDIR,
but LIBBPF_DIR is supplied by Makefile that include common.mk,
and rm command could be expanded to /root, if Makefile didn't
define this, which is too high risk. Thus, change the DESTDIR
name to "build" before, changing clean target.

I don't think this solves the issue #72, but it helps avoid
stale files in this DESTDIR.

Signed-off-by: Jesper Dangaard Brouer <[email protected]>

Makefile

@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
 
 LESSONS = $(wildcard basic*) $(wildcard packet*) $(wildcard tracing??-*)
+# LESSONS += advanced03-AF_XDP
 LESSONS_CLEAN = $(addsuffix _clean,$(LESSONS))
 
 .PHONY: clean $(LESSONS) $(LESSONS_CLEAN)

common/Makefile

@@ -6,7 +6,7 @@ all: common_params.o common_user_bpf_xdp.o common_libbpf.o
 CFLAGS := -g -Wall
 
 LIBBPF_DIR = ../libbpf/src/
-CFLAGS += -I$(LIBBPF_DIR)/root/usr/include/  -I../headers
+CFLAGS += -I$(LIBBPF_DIR)/build/usr/include/  -I../headers
 # TODO: Do we need to make libbpf from this make file too?
 
 common_params.o: common_params.c common_params.h

common/common.mk

@@ -38,7 +38,7 @@ EXTRA_DEPS +=
 # BPF-prog kern and userspace shares struct via header file:
 KERN_USER_H ?= $(wildcard common_kern_user.h)
 
-CFLAGS ?= -I$(LIBBPF_DIR)/root/usr/include/ -g
+CFLAGS ?= -I$(LIBBPF_DIR)/build/usr/include/ -g
 # Extra include for Ubuntu issue #44
 CFLAGS += -I/usr/include/x86_64-linux-gnu
 CFLAGS += -I../headers/
@@ -51,6 +51,7 @@ all: llvm-check $(USER_TARGETS) $(XDP_OBJ) $(COPY_LOADER) $(COPY_STATS)
 .PHONY: clean $(CLANG) $(LLC)
 
 clean:
+	rm -rf $(LIBBPF_DIR)/build
 	$(MAKE) -C $(LIBBPF_DIR) clean
 	$(MAKE) -C $(COMMON_DIR) clean
 	rm -f $(USER_TARGETS) $(XDP_OBJ) $(USER_OBJ) $(COPY_LOADER) $(COPY_STATS)
@@ -89,7 +90,7 @@ $(OBJECT_LIBBPF):
 		exit 1; \
 	else \
 		cd $(LIBBPF_DIR) && $(MAKE) all; \
-		mkdir -p root; DESTDIR=root $(MAKE) install_headers; \
+		mkdir -p build; DESTDIR=build $(MAKE) install_headers; \
 	fi
 
 # Create dependency: detect if C-file change and touch H-file, to trigger