The 'U' mode bit is removed in python 3.11. It has been
deprecated for a long time. The 'U' mode bit has no effect
so this change doesn't change any behavior.

See https://docs.python.org/3.11/whatsnew/3.11.html#changes-in-the-python-api

- Migrate pkg-config checks from atleast_version.
- Check ppoll with a fragment, since the function_name argument has been
  removed.

This is the last version on the 1.8.x branch. The build system will be
updated to the 1.9.x branch in a later commit.

This reverts commit cf3f8205c4.

It was a good initiative, but waf 2.0 introces backward incompatible
changes that break the pkg-config checks. The config checks will be
updated before migrating to waf 2.0.