[PATCH] selftests: lib.mk: fix out-of-tree builds

[Anders Roxell <anders.roxell@xxxxxxxxxx>]

Since commit ("selftests: error out if kernel header files are not yet
built") got merged, the kselftest build correctly because the
KBUILD_OUTPUT isn't set when building out-of-tree and specifying 'O='
This is the error message that pops up.

make --silent --keep-going --jobs=32 O=/home/anders/.cache/tuxmake/builds/1482/build INSTALL_PATH=/home/anders/.cache/tuxmake/builds/1482/build/kselftest_install ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- V=1 CROSS_COMPILE_COMPAT=arm-linux-gnueabihf- kselftest-install
make[3]: Entering directory '/home/anders/src/kernel/next/tools/testing/selftests/alsa'

-e error: missing kernel header files.
Please run this and try again:

    cd /home/anders/src/kernel/next/tools/testing/selftests/../../..
    make headers

make[3]: Leaving directory '/home/anders/src/kernel/next/tools/testing/selftests/alsa'
make[3]: *** [../lib.mk:77: kernel_header_files] Error 1

Fixing the issue by assigning KBUILD_OUTPUT the same way how its done in
kselftest's Makefile. By adding 'KBUILD_OUTPUT := $(O)' 'if $(origin O)'
is set to 'command line'. This will set the the BUILD dir to
KBUILD_OUTPUT/kselftest when doing out-of-tree builds which makes them
in its own separete output directory.

Signed-off-by: Anders Roxell <anders.roxell@xxxxxxxxxx>
---
 tools/testing/selftests/lib.mk | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tools/testing/selftests/lib.mk b/tools/testing/selftests/lib.mk
index b8ea03b9a015..d17854285f2b 100644
--- a/tools/testing/selftests/lib.mk
+++ b/tools/testing/selftests/lib.mk
@@ -44,6 +44,10 @@ endif
 selfdir = $(realpath $(dir $(filter %/lib.mk,$(MAKEFILE_LIST))))
 top_srcdir = $(selfdir)/../../..
 
+ifeq ("$(origin O)", "command line")
+  KBUILD_OUTPUT := $(O)
+endif
+
 ifneq ($(KBUILD_OUTPUT),)
   # Make's built-in functions such as $(abspath ...), $(realpath ...) cannot
   # expand a shell special character '~'. We use a somewhat tedious way here.
-- 
2.39.2