selftests/landlock: Fix out-of-tree builds

These changes simplify the Makefile and handle these 5 ways to build
Landlock tests:
- make -C tools/testing/selftests/landlock
- make -C tools/testing/selftests TARGETS=landlock gen_tar
- make TARGETS=landlock kselftest-gen_tar
- make TARGETS=landlock O=build kselftest-gen_tar
- make -C /tmp/linux TARGETS=landlock O=/tmp/build kselftest-gen_tar

This also makes $(KHDR_INCLUDES) available to other test collections
when building in their directory.

Fixes: f1227dc7 ("selftests/landlock: fix broken include of linux/landlock.h")
Fixes: 3bb267a3 ("selftests: drop khdr make target")
Cc: Anders Roxell <anders.roxell@linaro.org>
Cc: Guillaume Tucker <guillaume.tucker@collabora.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20220909103402.1501802-1-mic@digikod.net

tools/testing/selftests/landlock/Makefile

 # SPDX-License-Identifier: GPL-2.0
+#
+# First run: make -C ../../../.. headers_install
 
 CFLAGS += -Wall -O2 $(KHDR_INCLUDES)
+LDLIBS += -lcap
+
+LOCAL_HDRS += common.h
 
 src_test := $(wildcard *_test.c)
 
@@ -8,14 +13,10 @@ TEST_GEN_PROGS := $(src_test:.c=)
 
 TEST_GEN_PROGS_EXTENDED := true
 
-OVERRIDE_TARGETS := 1
-top_srcdir := ../../../..
-include ../lib.mk
-
-khdr_dir = $(top_srcdir)/usr/include
+# Static linking for short targets:
+$(TEST_GEN_PROGS_EXTENDED): LDFLAGS += -static
 
-$(OUTPUT)/true: true.c
-	$(LINK.c) $< $(LDLIBS) -o $@ -static
+include ../lib.mk
 
-$(OUTPUT)/%_test: %_test.c $(khdr_dir)/linux/landlock.h ../kselftest_harness.h common.h
-	$(LINK.c) $< $(LDLIBS) -o $@ -lcap -I$(khdr_dir)
+# Static linking for targets with $(OUTPUT)/ prefix:
+$(TEST_GEN_PROGS_EXTENDED): LDFLAGS += -static

tools/testing/selftests/lib.mk

@@ -42,6 +42,10 @@ endif
 selfdir = $(realpath $(dir $(filter %/lib.mk,$(MAKEFILE_LIST))))
 top_srcdir = $(selfdir)/../../..
 
+ifeq ($(KHDR_INCLUDES),)
+KHDR_INCLUDES := -isystem $(top_srcdir)/usr/include
+endif
+
 # The following are built by lib.mk common compile rules.
 # TEST_CUSTOM_PROGS should be used by tests that require
 # custom build rule and prevent common build rule use.