X_CFLAGS (set by AC_PATH_XTRA) was not used in Makefile.am. It was used in tests for X11/Dialog.h, etc. so configure worked, but make didn't.
DISTCLEANFILES = stamp-h
-AM_CPPFLAGS=-DINFODIR='"$(infodir)"' @CAIRO_CFLAGS@ @FRONTEND_CFLAGS@ -DSYSCONFDIR='"$(sysconfdir)"' \
+AM_CPPFLAGS=-DINFODIR='"$(infodir)"' @X_CFLAGS@ @CAIRO_CFLAGS@ @FRONTEND_CFLAGS@ -DSYSCONFDIR='"$(sysconfdir)"' \
-DLOCALEDIR='"$(localedir)"' -DSVGDIR='"$(svgdir)"' $(headers)
ACLOCAL_AMFLAGS = -I m4