Use of local-rust-root breaks build due to multiple candidates for libstd (and others) when there is a local rust install of the same version

[codyps]

Reproduction:

# Install the rustc snapshot cargo is using to /usr, and then:

./.travis.install.deps.sh
./configure --local-rust-root=rustc
make
# make fails

Full output: https://gist.github.com/c47a9a9251d0d4446e8f

Trimmed portion:

 Compiling libgit2-sys v0.0.1 (https://github.com/alexcrichton/git2-rs#0a97e473)
/home/cody/.cargo/git/checkouts/libz-sys-9674377d13f64fc8/master/src/lib.rs:1:1: 1:1 error: multiple rlib candidates for `std` found
/home/cody/.cargo/git/checkouts/libz-sys-9674377d13f64fc8/master/src/lib.rs:1 // Bindings coming soon to a theater near you!
                                                                              ^
/home/cody/.cargo/git/checkouts/libz-sys-9674377d13f64fc8/master/src/lib.rs:1:1: 1:1 note: candidate #1: /home/cody/g/cargo/rustc/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-4e7c5e5c.rlib
/home/cody/.cargo/git/checkouts/libz-sys-9674377d13f64fc8/master/src/lib.rs:1 // Bindings coming soon to a theater near you!
                                                                              ^
/home/cody/.cargo/git/checkouts/libz-sys-9674377d13f64fc8/master/src/lib.rs:1:1: 1:1 note: candidate #2: /usr/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-4e7c5e5c.rlib
/home/cody/.cargo/git/checkouts/libz-sys-9674377d13f64fc8/master/src/lib.rs:1 // Bindings coming soon to a theater near you!
                                                                              ^

Fixing this probably needs some change(s) in rustc to avoid choking.

[alexcrichton]

Closing as a dupe of rust-lang/rust#16402, but I believe this can be fixed by rust-lang/rust#19767.