Can't build for Mac OS X under XDarwin

[Randall Meadows]

I'm running XDarwin under Mac OS X.1.2, and am trying to get ddd to 
compile.  I've successfully built (and presumably installed) lesstif, 
but ddd won't completely build.

I'm issuing "sudo make install" from the ddd-3.3.1 directory; 
/usr/bin/ld generates some "unused multiple definitions" warnings, 
and then the line

+ cc -DHAVE_CONFIG_H -I. -I. -I. -I./.. -isystem /usr/X11R6/include 
-g -c -g ./ctest.c -o ctest.o

generates the error

cpp-precomp: could not open '/usr/X11R6/include'
make[2]: *** [ctest.o] Error 1
make[1]: *** [ctest.o] Error 2
make: *** [install-recursive] Error 1

/usr/X11R6/include most certainly exists on my system.  I'm pretty 
new to unix and building things from scratch...can anyone give me any 
pointers here?

Thanks very much!
randy