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
```

Install script of setuptools-6.1/pip-1.5.6 will create those links if needed

Add Makefile for 'make test'

Speed up `rbenv rehash`

This was introduced when merging the rbenv upstream change
(https://github.com/sstephenson/rbenv/commit/6bb7f07d2d).

Speed up rbenv by dynamically loading compiled command

It doesn't need to be a bash array and we don't need a separate index of
shims registered. Simply keep everything in a space-separated string and
use that as an index as well.

This assumes that executable names *never* have spaces in them.

On my system that has 25 versions under rbenv, this speeds up rehash
almost 3-fold:

- before: 391 ms
- after:  134 ms

This is achieved by removing duplicate names of executables before
registering them as shims. Since most Rubies will share a lot of the
same executable names ("ruby", "rake", "bundle", ...), this is a
considerable reduction in number of shims registered.

Too many of our users have a shell initialization set up that
inadvertently duplicates some or most of the entries in their PATH,
bringing the system paths again in front of rbenv's shims. If this was a
nested shell (a typical scenario when starting up tmux), `rbenv init`
would get eval'd again but this time, shims won't get added to the front
of the PATH and would only stay and the end of the path, effectively
rendering them useless.

I tried to argue that this is a user problem rather than rbenv's, but I
can't fix everybody shell init when they report bugs. Instead, let's
revert to simpler times in rbenv where we just roll along with the
duplication and don't ask any questions.

This reverts commit 03fa148e81.

Fixes #369

Delegate to `rbenv-shims` instead of `rbenv shims` and therefore skip
going through the main `rbenv` executable again that would set up a lot
of the environment that was already set.

Optimize pyenv-which: implement remove_from_path in Bash

This is the remaining part of
c69d9a1128.

    commit c69d9a1128
    Author: Mislav Marohnić <mislav.marohnic@gmail.com>
    Date:   Mon Oct 13 12:39:47 2014 +0200

        Isolate rbenv-which tests from any `.ruby-version` file on the system

        Having a `.ruby-version` file in any of the parent directories of the
        local clone of rbenv could cause the test suite to fail because it
        wasn't expecting a local version to be set.

This adds the missing parts from the upstream rbenv merge:
f4652fbbf0

- e4cbf04592
- 3ee395f9b5

Having a `.ruby-version` file in any of the parent directories of the
local clone of rbenv could cause the test suite to fail because it
wasn't expecting a local version to be set.

Fixes #533

Closes #560

This implements removing of the shims path element via bash
substitution, instead of jumping around in all the `$PATH` elements.

Running any shim (and thus `rbenv-exec`) would always execute
`rbenv-version-name` twice: once in `rbenv-exec` and another time in
`rbenv-which`, even though RBENV_VERSION variable would have already
been populated at this point.

Now RBENV_VERSION is respected within `rbenv-which`.

Fixes #626

The previous Makefile only worked on OS X. The dynamically generated
Makefile (from `Makefile.in`) should now work on multiple platforms
(tested on OS X and Ubuntu).

The `shobj-conf` script imported from bash seems to not support the
latest OS X. This makes sure that `SHOBJ_LDFLAG=-dynamiclib` is output
for Darwin10+ (latest version is Darwin 13.0).

Given the `-o <HOST-OS>` parameter, the script generates environment
variables with information how to compile dynamically loadable libraries
for that system.

Imported from bash-3.2.48

Seems to only work on OS X; fails on Travis
https://travis-ci.org/sstephenson/rbenv/jobs/16384866

It's slow and not necessary since we expect `$0` to already be expanded.

In tests this change forces us to deal with some relative paths, but
it's not a big deal. The `rbenv init -` output in the most common case
will be the same as before:

    source '/home/myuser/.rbenv/libexec/../completions/rbenv.bash'

This speeds up every `rbenv` invocation significantly.

With `realpath` extension, hooks tests on OS X will output
`/private/tmp` instead of `/tmp` because the latter is an actual symlink
to the former.

Avoid this mistmach in output assertions by expanding BATS_TMPDIR if
`realpath` extension is compiled.

On systems that support both C compiling and dynamic loading, we can
speed up `realpath()` (where most time in rbenv is spent) by replacing
it with a dynamically loaded bash builtin.

When `make -C src` is called in the project's root,
`libexec/rbenv-realpath.dylib` will be created. If it exists, rbenv will
attempt to load it as a builtin command. If it fails, execution will
fall back to the old `realpath()` shell function.