Buildworld error with read-only source tree

[Ryan Stone]

I just got the following error attempting to build r308430 with my source
tree on a read-only NFS mount.  I can work around it for now, but shouldn't
the source tree be untouched during a buildworld?

$ make -j4 buildworld buildkernel
--- buildworld ---
make[1]: "/repos/users/rstone/freebsd/Makefile.inc1" line 146:
SYSTEM_COMPILER: Determined that CC=cc matches the source tree.  Not
bootstrapping a cross-compiler.
--- buildworld_prologue ---
--------------------------------------------------------------
>>> World build started on Tue Nov  8 01:50:50 EST 2016
--------------------------------------------------------------
--- _worldtmp ---
--------------------------------------------------------------
>>> Rebuilding the temporary build tree
--------------------------------------------------------------
rm -rf /repos/users/rstone/freebsd/tmp
rm -rf /repos/users/rstone/freebsd/lib32
mkdir -p /repos/users/rstone/freebsd/tmp/lib
mkdir: /repos/users/rstone/freebsd/tmp: Read-only file system