tools/resolve_btfids: Use pkg-config to locate libelf

When libelf was not installed in the standard location, it cannot be
located by the current building config.

Use pkg-config to help locate libelf in such cases.

Signed-off-by: Shen Jiamin <shen_jiamin@comp.nus.edu.sg>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20221215044703.400139-1-shen_jiamin@comp.nus.edu.sg

tools/bpf/resolve_btfids/Makefile

@@ -56,13 +56,17 @@ $(BPFOBJ): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile) | $(LIBBPF_OU
 		    DESTDIR=$(LIBBPF_DESTDIR) prefix= EXTRA_CFLAGS="$(CFLAGS)" \
 		    $(abspath $@) install_headers
 
+LIBELF_FLAGS := $(shell $(HOSTPKG_CONFIG) libelf --cflags 2>/dev/null)
+LIBELF_LIBS  := $(shell $(HOSTPKG_CONFIG) libelf --libs 2>/dev/null || echo -lelf)
+
 CFLAGS += -g \
           -I$(srctree)/tools/include \
           -I$(srctree)/tools/include/uapi \
           -I$(LIBBPF_INCLUDE) \
-          -I$(SUBCMD_SRC)
+          -I$(SUBCMD_SRC) \
+          $(LIBELF_FLAGS)
 
-LIBS = -lelf -lz
+LIBS = $(LIBELF_LIBS) -lz
 
 export srctree OUTPUT CFLAGS Q
 include $(srctree)/tools/build/Makefile.include