Allow users to change the name of `install` target (#255)
Pass `--altinstall` to `ensurepip` (#255)
Use version suffixed executable (e.g. `python3.4`) for internal use of python (#255)
Stop creating symlinks w/ version suffix if `altinstall` has specified (#182, #255)
Calling `create_symlinks` after `ensurepip` is unnecessary (#182, #255)

Install script of setuptools-6.1/pip-1.5.6 will create those links if needed
Merge branch 'altinstall' (fixes #255)

Allow installing CPython with `make altinstall` instead of
`make install` with following command-line.

```sh
$ env PYTHON_MAKE_INSTALL_PACKAGE="altinstall" pyenv install -v 3.4.2
```