stubdom: make GMP aware that it's being cross-compiled

Append --build and --host flags to GMP's configure script so that it
knows it is being cross-compiled.

This should fix the issue that GMP doesn't compile with gcc 6, because
configure script won't try to test the host environment anymore.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>

stubdom/Makefile

@@ -171,7 +171,7 @@ gmp-$(XEN_TARGET_ARCH): gmp-$(GMP_VERSION).tar.bz2 $(NEWLIB_STAMPFILE)
 	rm $@ -rf || :
 	mv gmp-$(GMP_VERSION) $@
 	#patch -d $@ -p0 < gmp.patch
-	cd $@; CPPFLAGS="-isystem $(CROSS_PREFIX)/$(GNU_TARGET_ARCH)-xen-elf/include $(TARGET_CPPFLAGS)" CFLAGS="$(TARGET_CFLAGS)" CC=$(CC) $(GMPEXT) ./configure --disable-shared --enable-static --disable-fft --without-readline --prefix=$(CROSS_PREFIX)/$(GNU_TARGET_ARCH)-xen-elf
+	cd $@; CPPFLAGS="-isystem $(CROSS_PREFIX)/$(GNU_TARGET_ARCH)-xen-elf/include $(TARGET_CPPFLAGS)" CFLAGS="$(TARGET_CFLAGS)" CC=$(CC) $(GMPEXT) ./configure --disable-shared --enable-static --disable-fft --without-readline --prefix=$(CROSS_PREFIX)/$(GNU_TARGET_ARCH)-xen-elf --build=`gcc -dumpmachine` --host=$(GNU_TARGET_ARCH)-xen-elf
 	sed -i 's/#define HAVE_OBSTACK_VPRINTF 1/\/\/#define HAVE_OBSTACK_VPRINTF 1/' $@/config.h
 	touch $@