add the option --enable-local-rust to pull rust from your environment

rather than the snapshots.

make sure to get all of the files.

update to add nmatsakis' requested feature of pointing to a
different rustc install root.

usage: --enable-local-rust to enable
--local-rust-root="/path/to/rustc/" to change the path, which defaults to
"/usr/local/"

Tested on OS X and Linux, likely broken on windows.

src/etc/local_stage0.sh

@@ -0,0 +1,48 @@
+#!/bin/sh
+
+TARG_DIR=$1 
+PREFIX=$2
+
+BINDIR=bin
+LIBDIR=lib
+
+OS=`uname -s`
+case $OS in 
+    ("Linux"|"FreeBSD")
+	BIN_SUF=
+	LIB_SUF=.so
+	break
+	;;
+    ("Darwin")
+	BIN_SUF=
+	LIB_SUF=.dylib
+	break
+	;;
+    (*)
+	BIN_SUF=.exe
+	LIB_SUF=.dll
+	LIBDIR=bin
+	break
+	;;
+esac
+
+if [ -z $PREFIX ]; then
+    echo "No local rust specified."
+    exit 1
+fi
+
+if [ ! -e ${PREFIX}/bin/rustc ]; then
+    echo "No local rust installed at ${PREFIX}"
+    exit 1
+fi
+
+if [ -z $TARG_DIR ]; then
+    echo "No target directory specified."
+    exit 1
+fi
+
+cp ${PREFIX}/bin/rustc ${TARG_DIR}/stage0/bin/
+cp ${PREFIX}/lib/rustc/${TARG_DIR}/${LIBDIR}/* ${TARG_DIR}/stage0/${LIBDIR}/
+cp ${PREFIX}/lib/librust*${LIB_SUF} ${TARG_DIR}/stage0/${LIBDIR}/
+cp ${PREFIX}/lib/libcore*${LIB_SUF} ${TARG_DIR}/stage0/${LIBDIR}/
+cp ${PREFIX}/lib/libstd*${LIB_SUF} ${TARG_DIR}/stage0/${LIBDIR}/