ensure host compiler is used

dns/Makefile.in patch is derived from:
http://wiki.beyondlogic.org/patches/dhcp-4.3.0b1.bind_arm-linux-gnueabi.patch

This patch is already accepted upstream and will be included in the next
release:
--[snip]--
From Francis Dupont via RT <dhcp-suggest@isc.org>
To   rdkehn@yahoo.com 

Message body
It was fixed on the master branch sometimes ago.
Quoting master RELNOTES:

- Made the embedded bind libraries able to be cross compiled
  (please refer to the bind9 documentation to learn how to cross
  compile DHCP and its bind library dependency).
  [ISC-Bugs #38836]

This is in the Changes since 4.3.3 so for the next release.
--[snip]--

Signed-off-by: Doug Kehn <rdkehn@yahoo.com>

Index: dhcp-4.3.3-P1/bind/bind-9.9.7-P3/lib/export/dns/Makefile.in
===================================================================
--- dhcp-4.3.3-P1.orig/bind/bind-9.9.7-P3/lib/export/dns/Makefile.in
+++ dhcp-4.3.3-P1/bind/bind-9.9.7-P3/lib/export/dns/Makefile.in
@@ -168,7 +168,9 @@ code.h:	gen
 	./gen -s ${srcdir} > code.h
 
 gen: ${srcdir}/gen.c
-	${CC} ${ALL_CFLAGS} ${LDFLAGS} -o $@ ${srcdir}/gen.c ${LIBS}
+	${BUILD_CC} ${BUILD_CFLAGS} -I${top_srcdir}/lib/isc/include \
+		${BUILD_CPPFLAGS} ${BUILD_LDFLAGS} -o $@ ${srcdir}/gen.c \
+		${BUILD_LIBS}
 
 #We don't need rbtdb64 for this library
 #rbtdb64.@O@: rbtdb.c