Fix the pkgconfig file (issue JSBSim-Team#770)

Fix the stable release process.

Improvements to the build of wheels for Linux.

- Set the flag `-fno-math-errno` so that the `errno` flag is not any longer being set when math functions are called.
- Exclude using Cython version `0.29.26` for building Python wheels as it fails building wheels for PyPy38.

Fix the scripts to pass the XML validation.

Remove references to the Ubuntu Xenial release as it is no longer sup…

…ported by GitHub.

Always set useDefault to false when calling SGPropertyNode::tie()

See issue JSBSim-Team#363: setting `useDefault` to true is equivalent to have uninitialized variables.

Distutils no longer places double quotes around compiler flags (Pytho…

…n 3.9 for Windows.)

Build a source package for JSBSim Python module compatible with all p…

…latforms (Windows, MacOSX, Linux)

Until now, the CI workflow was building 2 different source packages (1 for Windows and 1 for MacOSX/Linux). Unfortunately, PyPI allows only one source package to be uploaded. This commit allows to produce just one source package that is compatible with all platforms.