Added support for cross compiling packages on x86_64.

Two new options for the configuration file were added:
 * XBPS_CROSS_TARGET
 * XBPS_CROSS_DIR

XBPS_CROSS_TARGET should be set to the target triplet that you build
with the mktoolchain script.

XBPS_CROSS_DIR should point to the cross directory that mktoolchain
created if you've built one.

As proof of concept the glibc32 package has been added for x86_64,
and it works perfectly even in the chroot! with glibc32 in place
that means that I can build a gcc multilib and use -m32 to build
32bit packages! OH YEAH I LOVE THAT SHIT!!!!

--HG--
extra : convert_revision : 6b0008865e084674a1c4b58266f681871e519c66

helpers/cross-compilation.sh

@@ -0,0 +1,39 @@
+#
+# This helper sets some required vars to be able to cross build
+# packages on xbps. The target is specified in the configuration file
+# and will be read any time the cross compilation flag is used.
+#
+[ -z "$XBPS_CROSS_TARGET" -o ! -d $XBPS_CROSS_DIR/bin ] && return 1
+
+# Check if all required bins are there.
+for bin in gcc g++ cpp ar as ranlib ld strip; do
+	if [ ! -x $XBPS_CROSS_DIR/bin/$XBPS_CROSS_TARGET-${bin} ]; then
+		msg_error "cross-compilation: cannot find ${bin}, aborting."
+	fi
+done
+
+SAVE_PATH="$PATH"
+
+cross_compile_setvars()
+{
+	export CC=$XBPS_CROSS_TARGET-gcc
+	export CXX=$XBPS_CROSS_TARGET-g++
+	export CPP=$XBPS_CROSS_TARGET-cpp
+	export AR=$XBPS_CROSS_TARGET-ar
+	export AS=$XBPS_CROSS_TARGET-as
+	export RANLIB=$XBPS_CROSS_TARGET-ranlib
+	export LD=$XBPS_CROSS_TARGET-ld
+	export STRIP=$XBPS_CROSS_TARGET-strip
+	export PATH="$XBPS_CROSS_DIR/bin:$PATH"
+}
+
+cross_compile_unsetvars()
+{
+	unset CC CXX CPP AR AS RANLIB LD STRIP PATH
+	export PATH="$SAVE_PATH"
+}
+
+if [ "$build_style" = "gnu_configure" ]; then
+	configure_args="--build=$XBPS_CROSS_HOST --host=$XBPS_CROSS_TARGET"
+	configure_args="$configure_args --target=$XBPS_CROSS_TARGET"
+fi