Define XML_POOR_ENTROPY in preparation of importing a new expat.

This is the tool fc-cache built as a native binary on the host
during the build.
The host might be non-netbsd, and we don't run configure, so we can't
know what kind of random functions it supports.

external/mit/xorg/tools/fc-cache/Makefile

@@ -1,4 +1,4 @@
-#	$NetBSD: Makefile,v 1.17 2019/07/20 03:54:50 mrg Exp $
+#	$NetBSD: Makefile,v 1.18 2019/09/16 06:16:42 maya Exp $
 
 .include <bsd.hostinit.mk>
 
@@ -119,6 +119,8 @@ HOST_CPPFLAGS+=	-I${FONTCONFIG} -I${FREETYPE} \
 		-I${DESTDIR}${X11INCDIR} -I. \
 		-I${FONTCONFIG}/../include
 
+HOST_CPPFLAGS+=	-DXML_POOR_ENTROPY
+
 HOST_CPPFLAGS+=	-DTOOL_FCCACHE
 
 LDADD=	-lz