mirror: Set DESTDIR to empty when building libbpf

If a value is assigned to the variable DESTDIR when compiling bpftool,
this variable will be passed to libbpf's Makefile.

In this case, libbpf's header files will be installed in an unexpected
directory, causing bpftool to be unable to find the header files.

Signed-off-by: jinzhiguang <jinzhiguang@kylinos.cn>

src/Makefile

@@ -42,15 +42,15 @@ $(LIBBPF_OUTPUT) $(BOOTSTRAP_OUTPUT) $(LIBBPF_BOOTSTRAP_OUTPUT) $(LIBBPF_HDRS_DI
 
 $(LIBBPF): $(wildcard $(BPF_DIR)/*.[ch] $(BPF_DIR)/Makefile) | $(LIBBPF_OUTPUT)
 	$(Q)$(MAKE) -C $(BPF_DIR) OBJDIR=$(patsubst %/,%,$(LIBBPF_OUTPUT)) \
-		PREFIX=$(LIBBPF_DESTDIR:/=) $(LIBBPF) install_headers
+		DESTDIR="" PREFIX=$(LIBBPF_DESTDIR:/=) $(LIBBPF) install_headers
 
 $(LIBBPF_INTERNAL_HDRS): $(LIBBPF_HDRS_DIR)/%.h: $(BPF_DIR)/%.h | $(LIBBPF_HDRS_DIR)
 	$(call QUIET_INSTALL, $@)
 	$(Q)install -m 644 -t $(LIBBPF_HDRS_DIR) $<
 
 $(LIBBPF_BOOTSTRAP): $(wildcard $(BPF_DIR)/*.[ch] $(BPF_DIR)/Makefile) | $(LIBBPF_BOOTSTRAP_OUTPUT)
 	$(Q)$(MAKE) -C $(BPF_DIR) OBJDIR=$(patsubst %/,%,$(LIBBPF_BOOTSTRAP_OUTPUT)) \
-		PREFIX=$(LIBBPF_BOOTSTRAP_DESTDIR:/=) \
+		DESTDIR="" PREFIX=$(LIBBPF_BOOTSTRAP_DESTDIR:/=) \
 		ARCH= CROSS_COMPILE= CC="$(HOSTCC)" LD="$(HOSTLD)" AR="$(HOSTAR)" $@ install_headers
 
 $(LIBBPF_BOOTSTRAP_INTERNAL_HDRS): $(LIBBPF_BOOTSTRAP_HDRS_DIR)/%.h: $(BPF_DIR)/%.h | $(LIBBPF_BOOTSTRAP_HDRS_DIR)