Build of several selftests fail if separate output directory is
specified by the following methods:
1) make -C tools/testing/selftests O=<build_dir>
2) export KBUILD_OUTPUT="build_dir"; make -C tools/testing/selftests

Build fails because of several reasons:
1) The kernel headers aren't found.
2) The path of output objects is wrong and hence unaccessible.

These problems can be solved by:
1) Including the correct path of uapi header files
2) By setting the BUILD variable correctly inside Makefile

Following different build scenarios have been tested after making these
changes to verify that nothing gets broken with these changes:
make -C tools/testing/selftests
make -C tools/testing/selftests/futex
make -C tools/testing/selftests/kvm
make -C tools/testing/selftests/landlock
make -C tools/testing/selftests/net
make -C tools/testing/selftests/net/mptcp
make -C tools/testing/selftests/vm
make -C tools/testing/selftests O=build
make -C tools/testing/selftests o=/opt/build
export KBUILD_OUTPUT="/opt/build"; make -C tools/testing/selftests
export KBUILD_OUTPUT="build"; make -C tools/testing/selftests
cd <any_dir>; make -C <src_path>/tools/testing/selftests
cd <any_dir>; make -C <src_path>/tools/testing/selftests O=build

---
Changes in V2:
        Revert the excessive cleanup which was breaking the individual
test build.

Muhammad Usama Anjum (10):
  selftests: set the BUILD variable to absolute path
  selftests: Add and export a kernel uapi headers path
  selftests: Correct the headers install path
  selftests: futex: Add the uapi headers include variable
  selftests: kvm: Add the uapi headers include variable
  selftests: landlock: Add the uapi headers include variable
  selftests: net: Add the uapi headers include variable
  selftests: mptcp: Add the uapi headers include variable
  selftests: vm: Add the uapi headers include variable
  selftests: vm: remove dependecy from internal kernel macros

 tools/testing/selftests/Makefile              | 32 +++++++++++++------
 .../selftests/futex/functional/Makefile       |  5 ++-
 tools/testing/selftests/kvm/Makefile          |  2 +-
 tools/testing/selftests/landlock/Makefile     |  2 +-
 tools/testing/selftests/net/Makefile          |  2 +-
 tools/testing/selftests/net/mptcp/Makefile    |  2 +-
 tools/testing/selftests/vm/Makefile           |  2 +-
 tools/testing/selftests/vm/userfaultfd.c      |  3 ++
 8 files changed, 32 insertions(+), 18 deletions(-)

-- 
2.30.2