selftests/bpf: Bring back c++ include/link test

Commit 5c26f9a78358 ("libbpf: Don't use cxx to test_libpf target")
converted existing c++ test to c. We still want to include and
link against libbpf from c++ code, so reinstate this test back,
this time in a form of a selftest with a clear comment about
its purpose.

v2:
* -lelf -> $(LDLIBS) (Andrii Nakryiko)

Fixes: 5c26f9a78358 ("libbpf: Don't use cxx to test_libpf target")
Signed-off-by: Stanislav Fomichev <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Acked-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/lib/bpf/.gitignore

···
 libbpf_version.h
 libbpf.pc
 FEATURE-DUMP.libbpf
-test_libbpf
 libbpf.so.*
 TAGS
 tags

tools/lib/bpf/Makefile

···
 VERSIONED_SYM_COUNT = $(shell readelf -s --wide $(OUTPUT)libbpf.so | \
 			      grep -Eo '[^ ]+@LIBBPF_' | cut -d@ -f1 | sort -u | wc -l)
 
-CMD_TARGETS = $(LIB_TARGET) $(PC_FILE) $(OUTPUT)test_libbpf
+CMD_TARGETS = $(LIB_TARGET) $(PC_FILE)
 
 all: fixdep
 	$(Q)$(MAKE) all_cmd
···
 
 $(OUTPUT)libbpf.a: $(BPF_IN_STATIC)
 	$(QUIET_LINK)$(RM) $@; $(AR) rcs $@ $^
-
-$(OUTPUT)test_libbpf: test_libbpf.c $(OUTPUT)libbpf.a
-	$(QUIET_LINK)$(CC) $(CFLAGS) $(LDFLAGS) $(INCLUDES) $^ -lelf -o $@
 
 $(OUTPUT)libbpf.pc:
 	$(QUIET_GEN)sed -e "s|@PREFIX@|$(prefix)|" \

tools/testing/selftests/bpf/.gitignore

···
 test_hashmap
 test_btf_dump
 xdping
+test_cpp
 /no_alu32
 /bpf_gcc

tools/testing/selftests/bpf/Makefile

···
 # Compile but not part of 'make run_tests'
 TEST_GEN_PROGS_EXTENDED = test_sock_addr test_skb_cgroup_id_user \
 	flow_dissector_load test_flow_dissector test_tcp_check_syncookie_user \
-	test_lirc_mode2_user xdping
+	test_lirc_mode2_user xdping test_cpp
 
 TEST_CUSTOM_PROGS = urandom_read
 
···
 		) > verifier/tests.h)
 $(OUTPUT)/test_verifier: test_verifier.c verifier/tests.h $(BPFOBJ) | $(OUTPUT)
 	$(CC) $(CFLAGS) $(filter %.a %.o %.c,$^) $(LDLIBS) -o $@
+
+# Make sure we are able to include and link libbpf against c++.
+$(OUTPUT)/test_cpp: test_cpp.cpp $(BPFOBJ)
+	$(CXX) $(CFLAGS) $^ $(LDLIBS) -o $@
 
 EXTRA_CLEAN := $(TEST_CUSTOM_PROGS)					\
 	prog_tests/tests.h map_tests/tests.h verifier/tests.h		\