Many people have had this problem (I've googled) but it's not in the bug database.
Building DDD 3.3.11 from source on Red Hat Enterprise Linux 4.2 with GCC 4.2.2 and using LessTif 0.95.0:
/usr/X11R6/include/X11/VendorP.h:87: error: previous declaration of 'VendorShellClassRec vendorShellClassRec' with 'C++' linkage
/usr/local/include/Xm/VendorSP.h:58: error: conflicts with new declaration with 'C' linkage
I would like to submit this as a bug and would like to know if there is a workaround.
Thanks!