enhancement: cmake option to install manual pages

[Corin-EU]

Thank you for reviving mp4v2 and also for providing the CMAKE build procedure.

Would you consider adding an enhancement to the CMAKE build procedure to build and install the manual pages?

A common way is to add an option WITH_feature_name and also a BUILD_ALL option which turns on all the other options -- in this case build and install the manual pages as well the library and tools,

[enzo1982]

Being able to generate the man pages using CMake would be nice. Can you create a PR for this?

Otherwise I'm not sure when I will be able to get to it - and I don't have a lot of CMake experience.

The man pages are generated using help2man, so CMake will have to check for its existence. Also the man pages can only be generated after the tools and when invoking the tools to get the input for help2man, care needs to be taken to use the correct libmp4v2, i.e. the one just build, not one already installed in the system.