Fix some dep errors with cross-compiled plugins

* Make sure plugins link to plugin dependencies, not target dependencies.
  Previously the --extern flag was being passed incorrectly but the dependency
  was being picked up by -L anyway.

* Fix a type and actually put the host dep directory into LD_LIBRARY_PATH, not
  the target directory. A test was added for this change.
auto merge of #282 : alexcrichton/cargo/fix-some-cross-compile-with-plugins, r=wycats

* Make sure plugins link to plugin dependencies, not target dependencies.
  Previously the --extern flag was being passed incorrectly but the dependency
  was being picked up by -L anyway.

* Fix a type and actually put the host dep directory into LD_LIBRARY_PATH, not
  the target directory. A test was added for this change.
Add a configure script

This configure script is similar to rust's in that it doesn't require anything
like autotools, it's just a meta-script to "generate" a makefile and perform
run-of-the-mill validation/discovery before the Makefile is run.

The main purpose of this rewrite is to support multi-target and targeted builds.
This will allow us to produce 32-bit snapshots for platforms as well as easily
providing `./configure --target $foo` for initialization.

cc #274
auto merge of #277 : alexcrichton/cargo/add-configure-script, r=wycats

This configure script is similar to rust's in that it doesn't require anything
like autotools, it's just a meta-script to "generate" a makefile and perform
run-of-the-mill validation/discovery before the Makefile is run.

The main purpose of this rewrite is to support multi-target and targeted builds.
This will allow us to produce 32-bit snapshots for platforms as well as easily
providing `./configure --target $foo` for initialization.

cc #274
Correctly preserve plugin deps

When copying files over from the old root to the new root on a fresh
compilation, care must be taken to preserve the correct plugin/host version of
each dependency. The previous code copied back over at most one library, but
this commit fixes this behavior by copying over all targets necessary for
compilation.
auto merge of #283 : alexcrichton/cargo/fix-fresh-plugin-deps, r=wycats

When copying files over from the old root to the new root on a fresh
compilation, care must be taken to preserve the correct plugin/host version of
each dependency. The previous code copied back over at most one library, but
this commit fixes this behavior by copying over all targets necessary for
compilation.