The problem appears to be that the configure script ends up with two instances of the same directory ({phpdirectory}sapi/nsapi) in the set of include directories and omits the include directory from the application server directory.
I worked around this with the following crude patch to the configure script: *** configure.orig Wed Mar 30 06:35:34 2005 --- configure Thu Apr 7 16:35:59 2005 *************** *** 7265,7271 **** case sapi/nsapi in "") ac_srcdir="$abs_srcdir/"; unset ac_bdir; ac_inc="-I. -I$abs_srcdir" ;; /*) ac_srcdir=`echo "sapi/nsapi"|cut -c 2-`"/"; ac_bdir=$ac_srcdir; ac_inc="-I$ac_bdir -I$abs_srcdir/$ac_bdir" ;; ! *) ac_srcdir="$abs_srcdir/sapi/nsapi/"; ac_bdir="sapi/nsapi/"; ac_inc="-I$ac_bdir -I$ac_srcdir" ;; esac --- 7265,7271 ---- case sapi/nsapi in "") ac_srcdir="$abs_srcdir/"; unset ac_bdir; ac_inc="-I. -I$abs_srcdir" ;; /*) ac_srcdir=`echo "sapi/nsapi"|cut -c 2-`"/"; ac_bdir=$ac_srcdir; ac_inc="-I$ac_bdir -I$abs_srcdir/$ac_bdir" ;; ! *) ac_srcdir="$abs_srcdir/sapi/nsapi/"; ac_bdir="sapi/nsapi/"; ac_inc="-I$NSAPI_INCLUDE -I$ac_srcdir" ;; esac