Configure script change to disable managing just the LLVM submodule

[larsbergstrom]

Right now, Servo is attempting to build Rust on Travis CI with a prebuilt LLVM. However, there's no configure flag to prevent Rust from still spending a huge amount of time doing git stuff with the LLVM module, as --disable-manage-submodules also prevents it from managing the others.

It would be great if, when you pass a --llvm-root, the configure script doesn't update the LLVM submodule.

[larsbergstrom]

@alexcrichton thanks - this is fantastic!