He mentioned bringing those files from Solaris; I'm tempted to guess they used cpp.
Backtrace is not too enlightening to me, but I haven't poked at xterm source since the late 1990s :)
haral:12093 Z$ LANG=en_US gdb =xterm
GNU gdb 6.3.50-20050815 (Apple version gdb-1708) (Thu Nov 3 21:59:02 UTC 2011)
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This GDB was configured as "x86_64-apple-darwin"...Reading symbols for shared libraries .......... done
(gdb) break _exit
Function "_exit" not defined.
Make breakpoint pending on future shared library load? (y or [n]) y
Breakpoint 1 (_exit) pending.
(gdb) run
Starting program: /opt/X11/bin/xterm
Reading symbols for shared libraries +++++++++.................................... done
Breakpoint 1 at 0x7fff890405c4
Pending breakpoint 1 - "_exit" resolved
Breakpoint 1, 0x00007fff890405c4 in _exit ()
(gdb) bt
#0 0x00007fff890405c4 in _exit ()
#1 0x000000010001d87f in Exit ()
#2 0x000000010000bc4f in VTparse ()
#3 0x000000010000b4f8 in VTRun ()
#4 0x000000010001b94e in main ()
(gdb)