error installing world from R/O src and obj

Attempting to install r353380 (amd64) with src and obj mounted via NFS.
The world has just been built albeit with NO_CLEAN=1.
Getting this:
...
===> lib/clang (install)
===> lib/clang/libllvm (install)
===> lib/clang/libclang (install)
===> lib/clang/liblldb (install)
===> lib/clang/headers (install)
clang-tblgen -gen-arm-fp16  -I
/usr/devel/svn/quick-head/contrib/llvm/tools/clang/include/clang/Basic -d
arm_fp16.h.d  -o arm_fp16.h
/usr/devel/svn/quick-head/contrib/llvm/tools/clang/include/clang/Basic/arm_fp16.td
clang-tblgen: error opening arm_fp16.h.d:Read-only file system
*** Error code 1

Stop.

The command is:
make installworld __MAKE_CONF=/dev/null SRCCONF=/dev/null DB_FROM_SRC=1 DESTDIR=/mnt

-- 
Andriy Gapon