[breaking changes] use Cargo features instead of the complicated build system.

This removes the whole `build` directory.
Istead of compiling the `ncollide3df32` or `ncollide2df64` projects,
compile the main `ncollide` projects with the appropriate feature
(2df32, 2df64, 3df32, 3df64, 4df32, 4df64) selected by your dependency
declaration of the `Cargo.toml` file, or using `--feature = "3df32"`
with your cargo build command line.