Ensure LD_LIBRARY_PATH is correct for plugins

At runtime rustc will dlopen() plugins, and if plugins have dynamic dependencies
they're likely to be in target/deps, so we need to make sure that directory is
in the right search path.
auto merge of #257 : alexcrichton/cargo/ld-lib-path, r=wycats

At runtime rustc will dlopen() plugins, and if plugins have dynamic dependencies
they're likely to be in target/deps, so we need to make sure that directory is
in the right search path.