Couple of bug fixes.

Allow `CONFIGURE_FLAGS` to be used with `give` for using things such as `--ninja` when configuring.

Use `PROMPT_COMMAND` to initialize the `PATH` environment variable, and also echo the current version (or tag/branch) being used. This will keep anything from possibly overwriting the `PATH`. The shells will also now open as a login shell, calling the normal chain of `.bashrc` files.