selftests: error out if kernel header files are not yet built

As per a discussion with Muhammad Usama Anjum [1], the following is how
one is supposed to build selftests:

    make headers && make -C tools/testing/selftests/mm

Change the selftest build system's lib.mk to fail out with a helpful
message if that prerequisite "make headers" has not been done yet.

[1] https://lore.kernel.org/all/bf910fa5-0c96-3707-cce4-5bcc656b6274@collabora.com/

[jhubbard@nvidia.com: abort the make process the first time headers aren't detected]
  Link: https://lkml.kernel.org/r/14573e7e-f2ad-ff34-dfbd-3efdebee51ed@nvidia.com
[anders.roxell@linaro.org: fix out-of-tree builds]
  Link: https://lkml.kernel.org/r/20230613074931.666966-1-anders.roxell@linaro.org
Link: https://lkml.kernel.org/r/20230606071637.267103-12-jhubbard@nvidia.com
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Tested-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/Makefile

@@ -145,10 +145,12 @@ ifneq ($(KBUILD_OUTPUT),)
   abs_objtree := $(realpath $(abs_objtree))
   BUILD := $(abs_objtree)/kselftest
   KHDR_INCLUDES := -isystem ${abs_objtree}/usr/include
+  KHDR_DIR := ${abs_objtree}/usr/include
 else
   BUILD := $(CURDIR)
   abs_srctree := $(shell cd $(top_srcdir) && pwd)
   KHDR_INCLUDES := -isystem ${abs_srctree}/usr/include
+  KHDR_DIR := ${abs_srctree}/usr/include
   DEFAULT_INSTALL_HDR_PATH := 1
 endif
 
@@ -162,7 +164,7 @@ export KHDR_INCLUDES
 # all isn't the first target in the file.
 .DEFAULT_GOAL := all
 
-all:
+all: kernel_header_files
 	@ret=1;							\
 	for TARGET in $(TARGETS); do				\
 		BUILD_TARGET=$$BUILD/$$TARGET;			\
@@ -173,6 +175,23 @@ all:
 		ret=$$((ret * $$?));				\
 	done; exit $$ret;
 
+kernel_header_files:
+	@ls $(KHDR_DIR)/linux/*.h >/dev/null 2>/dev/null;                          \
+	if [ $$? -ne 0 ]; then                                                     \
+            RED='\033[1;31m';                                                  \
+            NOCOLOR='\033[0m';                                                 \
+            echo;                                                              \
+            echo -e "$${RED}error$${NOCOLOR}: missing kernel header files.";   \
+            echo "Please run this and try again:";                             \
+            echo;                                                              \
+            echo "    cd $(top_srcdir)";                                       \
+            echo "    make headers";                                           \
+            echo;                                                              \
+	    exit 1;                                                                \
+	fi
+
+.PHONY: kernel_header_files
+
 run_tests: all
 	@for TARGET in $(TARGETS); do \
 		BUILD_TARGET=$$BUILD/$$TARGET;	\