configure: Add an option to use the cargo build system

This commit adds a `--enable-rustbuild` option to the configure script which
will copy a different `Makefile.in` into place to intercept all `make`
invocations.

Currently this makefile only has one target, but it's expected to be filled out
quite a bit over time!

src/bootstrap/mk/Makefile.in

@@ -0,0 +1,23 @@
+# Copyright 20126 The Rust Project Developers. See the COPYRIGHT
+# file at the top-level directory of this distribution and at
+# http://rust-lang.org/COPYRIGHT.
+#
+# Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+# http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+# <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+# option. This file may not be copied, modified, or distributed
+# except according to those terms.
+
+include config.mk
+include $(CFG_SRC_DIR)mk/util.mk
+
+ifdef VERBOSE
+BOOTSTRAP_ARGS := -v
+else
+BOOTSTRAP_ARGS :=
+endif
+
+BOOTSTRAP := $(CFG_PYTHON) $(CFG_SRC_DIR)src/bootstrap/bootstrap.py $(BOOTSTRAP_ARGS)
+
+all:
+	$(Q)$(BOOTSTRAP)