Correct handling of sysconfig._BASE_PREFIX in cross environments.

[freakboy3742]

Ensure that sysconfig._BASE_PREFIX is bound to base_prefix, not prefix.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[mhsmith]

FYI: when getting pybind11 working on Android, I encountered the same issue which led you to override sysconfig._BASE_PREFIX on iOS: that it's used to generate the value of sysconfig.get_path("include").

However, I'm going to try only overriding sysconfig._BASE_[EXEC_]PREFIX and not sys.base_[exec_]prefix, because the latter might have other undesired effects, redirecting things to the target Python which really should be for the build Python.