Auto generate Windows import libraries when using a pyo3 config file

[messense]

Currently the build script of pyo3-ffi calls pyo3-build-config's resolve_interpreter_config function

pyo3/pyo3-ffi/build.rs

Lines 79 to 80 in 87bd10c

	fn configure_pyo3() -> Result<()> {
	let interpreter_config = resolve_interpreter_config()?;

when PYO3_CONFIG_FILE env var is set, it simply read the file using InterpreterConfig::from_reader which missed the "generate import library" setup thus no python3.dll/python3y.dll is generated causing linking errors

pyo3/pyo3-build-config/src/lib.rs

Lines 179 to 180 in 87bd10c

	if !CONFIG_FILE.is_empty() {
	InterpreterConfig::from_reader(Cursor::new(CONFIG_FILE))

[messense]

cc @ravenexp

[ravenexp]

LGTM in general, but I'd fix up lib_name either way.

[ravenexp]

Looks good, thanks!

[davidhewitt]

👍