build fails on read-only /usr/src

[George Michaelson <ggm%apnic.net@localhost>]

echo '#include <initscan.c>' >/usr/src/external/bsd/flex/dist/scan.c
sh: cannot create /usr/src/external/bsd/flex/dist/scan.c: read-only  
file system

*** Failed target:  scan.c
*** Failed command: echo '#include <initscan.c>' >/usr/src/external/ 
bsd/flex/dist/scan.c
*** Error code 2

Stop.
nbmake: stopped in /usr/src/tools/lex

I have invoked

nice ./build.sh -U -u -x -O /usr/obj release &>/var/tmp/build.log

looks like somebody has modified src state, in a way which demands a  
write in /usr/src which hasn't been OBJified

-G