I have a use case in PyOxidizer where I want to use the
pyo3-build-config crate as a library crate so I can access the
`InterpreterConfig` struct so I can read/write config files without
reinventing the wheel.
This is doable before this commit. But it requires that the
build environment have a Python interpreter. This is undesirable
for library usage.
This commit introduces a cargo feature flag to control whether the
build script does anything. The feature flag must be present for
the build script to resolve a config. The feature flag is enabled
by default for backwards compatibility. The pyo3 and pyo3-macros-backend
crates use this feature by default, for backwards compatibility and
because it is the reasonable default.
This is probably room to conditionalize some APIs and other behavior
based on this feature flag. But we stop short of doing that for
the time being.
