Python 3.8 has different flags for pyembed, needs --embed

As recommended in the docs, to support both 3.8 and previous versions, we try to use python3-config --embed and fallback to the previous behavior.

Fixes https://gitlab.com/ita1024/waf/issues/2239

See https://docs.python.org/dev/whatsnew/3.8.html#debug-build-uses-the-same-abi-as-release-build