Allow lazy creation of the DSOs list, which
can avoid unnecessary (~slow) probing work
when running eg. './setup.py sdist'.

Helps with GHA, where conditional values are easier than
conditionally unset.

Maybe simplify/speed up PIP installs

More convenient to override the whole platform name,
including architecture.

* probe:
  handle strings in MSVC object files
  add toolchain inspection

This small change fixes a problem when a common list object is used in setup.py to declare dependencies for multiple extension objects. In this case the library dependencies get accumulated in the same list which causes problems in more complex builds.

Since _dsoinfo.py is "standard" convention,
only allow specific override (or omission).