Use C++20 modules

[taminob]

The main motivation for the option PPPLUGIN_ENABLE_CPP17_COMPATIBILITY is that the library should have C++20 module support.

This issue is about introducing this support if PPPLUGIN_ENABLE_CPP17_COMPATIBILITY is not specified.
A part of this issue is also to improve this option so that the headers do not depend on this option being set in the build system. This could be achieved by generating a header file with according C preprocessor macros.