Problem locating include file

I'm working with NSPR on Windows XP, using MozillaBuild 1.3 and Visual 
Studio 2005.  I've added a check to the configure script using 
AC_DEFUN[...], and the check tries to compile and run a small piece of 
code.  I'm having trouble telling the compiler how to find one of the 
include files needed for the code.

I've set CPPFLAGS to specify the include directory using the -I compiler 
option, but for some reason the compiler still can't find the file.  The 
only way I've been able to get it to work is either to place the include 
file in the same directory from which I'm running the configure script, 
or to add the include file's path to the INCLUDE environment variable.

Is there some way to get the -I option to work as expected?  Thanks for 
any help.

- Jon Leighton