That’s one of my concerns 
That would mean that people who do expect those things to work would be installing it. And “work” means IDEs know about it, means the IT department (or security-conscious team member) approves of it, and many more things that we can reasonably take care of with a system package.
You’re the only person on this thread who can actually look at what I did 
Basically, the pkg-config files need hacking, and you need to specify LD_LIBRARY_PATH to run it if you --enable-shared (which I do, but I guess that’s not required). Anything that embeds the prefix path needs updating (but I think this is just ensurepip and possibly pydoc_data). I think sysconfig ignores all the path info in the bundled Makefile, so that should be fine.
And of course you need all the system binaries to match perfectly. 
No it’s not… It’s a terrible idea from the POV of working on a particular platform and obtaining security patches. But because of the strict ABI dependency, there isn’t really a choice.
Either we bundle, or the user builds on their target platform (or downloads a platform-specific build from someone who built on the target platform).