--with-tcl won't gmake

I'm trying to install pg with the --with-tcl and --with-perl options on
an HP-UX 11.0 system. ./configure runs okay, but when I run gmake I get
the following:

In file included from /usr/local/include/tk.h:83,
from pgtkAppInit.c:15:
/usr/local/lib/gcc-lib/hppa2.0n-hp-hpux11.00/3.1/include/X11/Xlib.h:67:19:
X11/X.h: No such file or directory
/usr/local/lib/gcc-lib/hppa2.0n-hp-hpux11.00/3.1/include/X11/Xlib.h:70:28:
X11/Xfuncproto.h: No such file or directory
/usr/local/lib/gcc-lib/hppa2.0n-hp-hpux11.00/3.1/include/X11/Xlib.h:71:25:
X11/Xosdefs.h: No such file or directory
In file included from /usr/local/include/tk.h:83,
from pgtkAppInit.c:15:
/usr/local/lib/gcc-lib/hppa2.0n-hp-hpux11.00/3.1/include/X11/Xlib.h:205:
parse error before "Pixmap"
/usr/local/lib/gcc-lib/hppa2.0n-hp-hpux11.00/3.1/include/X11/Xlib.h:209:
parse error before "font"
/usr/local/lib/gcc-lib/hppa2.0n-hp-hpux11.00/3.1/include/X11/Xlib.h:214:
parse error before "clip_mask"
/usr/local/lib/gcc-lib/hppa2.0n-hp-hpux11.00/3.1/include/X11/Xlib.h:217:
parse error before '}' token
/usr/local/lib/gcc-lib/hppa2.0n-hp-hpux11.00/3.1/include/X11/Xlib.h:239:
parse error before "VisualID"
/usr/local/lib/gcc-lib/hppa2.0n-hp-hpux11.00/3.1/include/X11/Xlib.h:248:
parse error before '}' token
/usr/local/lib/gcc-lib/hppa2.0n-hp-hpux11.00/3.1/include/X11/Xlib.h:256:
parse error before "Visual"
/usr/local/lib/gcc-lib/hppa2.0n-hp-hpux11.00/3.1/include/X11/Xlib.h:270:
parse error before "Window"

plus a few hundred more similar lines, and then I get this:

In file included from pgtkAppInit.c:15:
/usr/local/include/tk.h:107: syntax error before "typedef"
/usr/local/include/tk.h:583: parse error before "Window"
/usr/local/include/tk.h:585: parse error before "subwindow"
/usr/local/include/tk.h:586: parse error before "time"
/usr/local/include/tk.h:589: conflicting types for `state'
/usr/local/lib/gcc-lib/hppa2.0n-hp-hpux11.00/3.1/include/X11/Xlib.h:889:
previous declaration of `state'
/usr/local/include/tk.h:592: parse error before '}' token
/usr/local/include/tk.h:599: parse error before "Window"
/usr/local/include/tk.h:601: parse error before "XActivateEvent"
/usr/local/include/tk.h:602: parse error before "XDeactivateEvent"

followed by a few hundred similar lines. I've got the tcl and tk
packages from the HP Porting and Archive Center installed; I even tried
installing the tcltk package on the chance that the individual packages
had missed something. Anyone know why I'm getting these errors and
subsequently failing? I might add that if I compile without --with-tcl
it works just fine. TIA.

Petre