Document default build configuration customizations (#2911)

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@ -98,6 +98,47 @@ custom definition files.
[definitions]: https://github.com/pyenv/pyenv/tree/master/plugins/python-build/share/python-build
### Default build configuration
Without the user customizing the build with environment variables (see below),
`python-build` builds Python with mostly default Configure options
to maintain the principle of the least surprise.
The exceptions -- non-default options that are set by default -- are listed below:
| Option/Behavior | Rationale |
|-----------------|-----------|
| `--enable-shared` is on by default. Pass `--disable-shared` to Configure options to override | The official CPython Docker image uses it. It's required to embed CPython. |
| argument to `--enable-universalsdk` is ignored and set to `/` |
| `--with-universal-archs` defaults to `universal2` on ARM64 architecture | the only dual-architecture Macs in use today are Apple Silicon which can only build that one |
| argument to `--enable-framework` is ignored and set to a specific value | CPython's build logic requires a very specific argument to avoid installing the `Applications` part globally |
| argument to `--enable-unicode` in non-MacOS is overridden to `ucs4` for 2.x-3.3 |
| `MACOSX_DEPLOYMENT_TARGET` defaults to the running MacOS version |
#### Integration with 3rd-party package ecosystems
##### Homebrew
In MacOS, Homebrew is used to find dependency packages if `brew` is found on `PATH`.
Set `PYTHON_BUILD_SKIP_HOMEBREW` to avoid using it.
When Homebrew is used, its `include` and `lib` paths are added to compiler search path (the latter is also set as `rpath`),
and also Python dependencies that are typically keg-only are searched for in the Homebrew installation and added individually.
**NOTE:** Homebrew is not used in Linux by default because it's rolling-release which causes a problem.
Upgrading a Python dependency in Homebrew to a new major version (that `brew` does without warning)
would break all Pyenv-managed installations that depend on it.
You can use a [community plugin `fix-version`](https://github.com/pyenv/pyenv/wiki/Plugins#community-plugins)
to fix installations in such a case.
##### Portage
In FreeBSD, if `pkg` is on PATH, Ports are searched for some dependencies that Configure is known to not search for via `pkg-config`.
(Later versions of CPython search for more packages via `pkg-config` so this may eventually become redundant.)
### Special environment variables
You can set certain environment variables to control the build process.