Add --target option for cross builds

"dpkg-buildpackage -aarmel -b" ends with:

 debian/rules build
mkdir -p build
cd build && ../configure --prefix=/usr --enable-cxx --enable-mpbsd --enable-fat --build x86_64-linux-gnu --host arm-linux-gnueabi \
            AR= arm-linux-gnueabi-ar CC=" arm-linux-gnueabi-gcc" CFLAGS="-Wall -g -D_REENTRANT -O3 -Wa,--defsym,USES_THUMB=1" \
            CXX=" arm-linux-gnueabi-g++" CXXFLAGS="-g -O2"
configure: WARNING: you should use --build, --host, --target
configure: error: --target is not appropriate for GMP
Use --build=CPU-VENDOR-OS if you need to specify your CPU and/or system
explicitly. Use --host if cross-compiling (see "Installing GMP" in the
manual for more on this).
make: *** [configure-stamp] Error 1
dpkg-buildpackage: error: debian/rules build gave error exit status 2

So I added --target option in debian/rules to pass that:

18:24 hrw@home:debian$ diff rules.orgi rules -u
--- rules.orgi 2010-09-22 18:24:37.806068998 +0200
+++ rules 2010-09-22 18:24:40.836068986 +0200
@@ -52,7 +52,7 @@
   confflags_build += --build $(DEB_BUILD_GNU_TYPE)
 else
   AR = $(DEB_HOST_GNU_TYPE)-ar
- confflags_build += --build $(DEB_BUILD_GNU_TYPE) --host $(DEB_HOST_GNU_TYPE)
+ confflags_build += --build $(DEB_BUILD_GNU_TYPE) --host $(DEB_HOST_GNU_TYPE) --target $(DEB_HOST_GNU_TYPE)
 endif

 ifneq (,$(findstring $(DEB_HOST_ARCH), amd64 ppc64 kfreebsd-amd64))

And it builds. There is a problem with libtool so it does not finish but with this fix it at least passes configure step.