Found a solution for this over on the omnetpp google group, I'll paste it in here:
Hi all,
just tried Mountain Lion, which was released yesterday. Unfortunately,
the new MacOSX does not include X11 by default. I really HATE this
decision. As a result, it does not pass the Tcl/Tk checks when we do
./configure.
After hours of efforts, I finally figured out how to work around this:
- download and install XQuartz at http://xquartz.macosforge.org/landing/
- make a symbolic link at /usr/include: sudo ln -s /opt/X11/include/X11 /usr/include
and you are all set. In 2, you may also set some environment variables
accordingly instead of making the symbolic link.
Future OMNet++ releases may consider setting associated environment
variables so that the compiler looks for Xlib.h in
/opt/X11/include/X11 with Mountain Lion.
Just successfully compiled the simulator and haven't encountered other
problems yet.
Have fun,
TS