Rename `$OUT_DIR/pyo3-cross-compile-config.txt` to
`$OUT_DIR/<triple>/pyo3-build-config.txt` to exclude the possibility
of using stale build configuration data when the build target changes.
Use the presence of the corresponding build configuration file
in the `pyo3-build-config` build script output directory
to detect whether we are cross compiling or not.
This patch enables cross compilation without using
any of `PYO3_CROSS_*` env variables in many cases.
Try to generalize `windows_hardcoded_cross_compile()`
to all supported target platforms (when possible).
Rename it to `default_cross_compile()` and add some unit tests.
Rewrite `load_cross_compile_config()` to fall back to
the default interpreter configuration when no other config
information sources are available.
Change the `CrossCompileConfig` structure definition and make
the public `lib_dir` field optional to support more flexible
cross-compilation configuration in the future.
FIXME: This change breaks the public `pyo3-build-config` crate API.
Update the sysconfigdata extraction functions to fall through
when `lib_dir` field is not set.
WIP: Add `unwrap()` stubs to the main cross compile switch.
Add a new public crate function `cross_compile_from_to()` using
`target_lexicon::Triple` arguments instead of plain strings
used in `cross_compile()`.
Deprecate `pyo3_build_config::cross_compile()` since v0.17.
Attempt to extract common code patterns into methods and standalone
helper functions. Add docstrings to the new private items.
Make some of the new helper functions public within the PyO3 crate
and reuse them in the build scripts.
Add PYO3_CROSS_PYTHON_VERSION parsing unit test.
Add a ChangeLog entry mentioning the new `pyo3-build-config` API.
Python 3.6 and older are not supported by the current PyO3 version,
so the removed interpreter version check was a no-op.
`pyo3_build_config::get()` attempts to read a config file from disk
when PyO3 is cross-compiling, which is probably bad for rust-analyzer
and other IDEs that attempt to sandbox the proc macro code.
