Newbie can't get gcc to find include files

[Gamaliel]

gcc is not finding an include file that is present when I use Cygwin
and a bash shell.  I have written a simple hello world program to
illustrate the problem.
===================================
#include <stdio.h>
#include </usr/X11R6/include/Xm/Xm.h>

void main()
{
  printf "Hello World!\n";
}
===================================

I compile the program as follows

gcc hello.c -o hello -lXm

I then get error messages such as

/usr/X11R6/include/Xm/Xm.h:32: X11/Intrinsic.h: No such file or
directory.

Intrinsic.h is included in Xm.h as follows

#include <X11/Intrinsic.h>

When I look for Intrinsic.h in /usr/X11R6/include/Xm/X11 it is
present.

The files are specified to be in my path in the .profile directory.

PATH="/usr/X11R6/include/Xm/X11:$PATH"
export PATH

So why doesn't gcc know that it is present?