configure message re: rustbuild assumes curr dir is source dir

[pnkfelix]

If you build rust out of tree, the configure script currently prints a message:

onfigure: NOTE you have now configured rust to use a rewritten build system
configure:      called rustbuild, and as a result this may have bugs that 
configure:      you did not see before. If you experience any issues you can
configure:      go back to the old build system with --disable-rustbuild and
configure:      please feel free to report any bugs!
configure: 
configure: run `python x.py --help`

The problem is that if I am running configure in a directory other than the root of the source directory, then the instruction to run python x.py --help will not work.

Furthermore, as a user who has often encountered software not supporting out-of-tree builds, when I encounter a message like that, my first instinct is to assume that this software does not support out of tree builds.

I recommend that the configure script detect whether the current directory is not the root of the Rust source directory, and if so, use ${CFG_SRC_DIR}/x.py as the path to the python script instead of just x.py.

[pnkfelix]

cc @alexcrichton

[petrochenkov]

It probably makes sense to just copy x.py into the build directory on first run so it could be invoked without any prefixes.

[pnkfelix]

@petrochenkov oh indeed that may make sense, since I just noticed that the Makefile.in also assumes that x.py is present in the build directory...