I managed to build somehow devel/kbuild under -current - after getting
kmk: Leaving directory `/usr/pkgsrc/devel/kbuild/work/kBuild'
echo done > /usr/pkgsrc/devel/kbuild/work/kBuild/out/netbsd.amd64/release/bootstrap/ts-stage2-install
./kBuild/env.sh: info: Executing command: kmk NIX_INSTALL_DIR=/usr/pkg BUILD_TYPE=release MY_INST_MODE=644 MY_INST_BIN_MODE=755 rebuild
Config.kmk:78: Neither SvnInfo nor .svn/* was found in the root. Will have to cook up something too keep the build happy.
/usr/pkgsrc/devel/kbuild/work/kBuild/src/Makefile.kmk:30: /usr/pkgsrc/devel/kbuild/work/kBuild/sed/Makefile.kmk: No such file or directory
/usr/pkgsrc/devel/kbuild/work/kBuild/src/Makefile.kmk:31: /usr/pkgsrc/devel/kbuild/work/kBuild/kmk/Makefile.kmk: No such file or directory
/usr/pkgsrc/devel/kbuild/work/kBuild/src/Makefile.kmk:32: /usr/pkgsrc/devel/kbuild/work/kBuild/kash/Makefile.kmk: No such file or directory
/usr/pkgsrc/devel/kbuild/work/kBuild/src/Makefile.kmk:33: /usr/pkgsrc/devel/kbuild/work/kBuild/kDepPre/Makefile.kmk: No such file or directory
/usr/pkgsrc/devel/kbuild/work/kBuild/src/Makefile.kmk:34: /usr/pkgsrc/devel/kbuild/work/kBuild/kObjCache/Makefile.kmk: No such file or directory
/usr/pkgsrc/devel/kbuild/work/kBuild/src/Makefile.kmk:35: /usr/pkgsrc/devel/kbuild/work/kBuild/misc/Makefile.kmk: No such file or directory
kmk: *** No rule to make target `/usr/pkgsrc/devel/kbuild/work/kBuild/misc/Makefile.kmk'. Stop.
./kBuild/env.sh: info: rc=2: kmk NIX_INSTALL_DIR=/usr/pkg BUILD_TYPE=release MY_INST_MODE=644 MY_INST_BIN_MODE=755 rebuild
(the missing files are present in another directory in different level, some environment variable not initialized?)
and then following a long list of files not found where they are expected (but present in the tree), basically just by copying or linking the files or folders in question.
Then I tried to go through the hoops and see how VirtualBox builds. I used 5.1.30 (there is a problem with 5.20 and bridging over wifi - it doesn't work, at least under Windows, so I stayed secure and went back a bit). I managed to coerce the configure script into successful completion, but then the trouble with many files/folders being off by one level or so was too much to fight with.
So yes, it does seem to be a bit more involved. I had in the past downloaded and used the 7.0 binaries uwe@ had offered earlier and was hoping to get somewhere with -current, but frankly it seems too much...