Read-only /usr/ports (problem with fontconfig)

[Cory Bajus]

Is a read-only /usr/ports directory supported?

I received the following error message during installation of
x11-fonts/fontconfig (as part of a build of x11/XFree86-4):

 install -c -o root -g wheel -m 444 ./fontconfig.pc /usr/X11R6/libdata/pkgconfig
/fontconfig.pc
gmake[2]: Leaving directory `/usr/work/usr/ports/x11-fonts/fontconfig/work/fontc
onfig-2.1.92'
gmake[1]: Leaving directory `/usr/work/usr/ports/x11-fonts/fontconfig/work/fontc
onfig-2.1.92'
===>   Generating temporary packing list
cannot create /usr/ports/x11-fonts/fontconfig/pkg-install: read-only file system
*** Error code 2
Stop in /usr/ports/x11-fonts/fontconfig.
*** Error code 1
Stop in /usr/ports/x11/XFree86-4-libraries.
*** Error code 1
Stop in /usr/ports/x11/XFree86-4.

My /usr/ports directory is NFS mounted (read-only) from another
system.  I have the following entries in /etc/make.conf to allow
multiple clients to share the same /usr/ports tree:

WRKDIRPREFIX=   /usr/work      (local directory)
DISTDIR=        /usr/distfiles (NFS mounted read-write)

Is it normal for a port to try to write into /usr/ports when
WRKDIRPREFIX is defined like this?

Thanks in advance,

Cory.