Build error

[Sad Clouds <cryintothebluesky%googlemail.com@localhost>]

With NetBSD sources mounted read-only, I get the following error. Is there a 
requirement to have read/write access to source files when building -current? 

#      yacc  libcvs//opt/src/external/gpl2/xcvs/dist/lib/getdate.c
/opt/tools.amd64/bin/nbyacc      -o 
/opt/src/external/gpl2/xcvs/dist/lib/getdate.c 
/opt/src/external/gpl2/xcvs/dist/lib/getdate.y

...

/opt/tools.amd64/bin/nbyacc: f - cannot open 
"/opt/src/external/gpl2/xcvs/dist/lib/getdate.c"
*** [getdate.c] Error code 2
nbmake: stopped in /opt/src/external/gpl2/xcvs/lib/libcvs