Includes:
- fix python warnings and wrap pkgconfig_interface_include_directories in quotes to treat it as a string
- prevent failure when Python is not found
- refactor python bindings checks and error messages
- export 3rd-party include directories and linked libraries
- Adding a NS3_REEXPORT_THIRD_PARTY_LIBRARIES option to always export include directories and libraries used by a module, making it automatically available to 3rd-party modules
- Making NS3_REEXPORT_THIRD_PARTY_LIBRARIES option enabled by default
- replace colliding shortcuts with list of non-ambiguous shortcuts
- remove unnecessary code blocks from click
- warn users that doxygen target requires examples and tests
- These are used to generate introspected documentation and guarantee proper dependency checking and build order
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