Building -current incrementally.

So I installed 7.5-stable on a VPS and that kernel panicked during installation
several times. I then grabbed a 7.6-current image and installed the system
using that. That kernel does not panic so I am running -current on that system
since then. Checked out /usr/src, build the system from source using
make build. Did cvs -q update -dP -A every now and then and just ran
make followed by doas make install from time to time in the various directories.
Accidentally ran make in /usr/src and got an unexpected Permission denied
error. I would have expected to be able to run make in /usr/src without any
permission issues. Is this a bug in the build system? There must be a way to
build base incrementally without having to rebuild everything.
$cd /usr/src && make currently stops with a Permission denied error I would not
have expected to happen.


build: atu-intersil-int: Permission denied
*** Error 1 in sys/dev/microcode/atmel (Makefile:22 'atu-intersil-int')
*** Error 2 in sys/dev/microcode (<bsd.subdir.mk>:48 'all': @for entry in afb atmel bnx cirruslogic fxp kue neomagic myx ral rsu rtwn rum s...)
*** Error 2 in sys (<bsd.subdir.mk>:48 'all': @for entry in dev/microcode arch/alpha arch/amd64 arch/arm64 arch/armv7 arch/hppa arch/i386 ...)
*** Error 2 in /usr/src (<bsd.subdir.mk>:48 'all': @for entry in lib include bin libexec sbin usr.bin usr.sbin share games gnu sys; do set ...)