Add a deprecated macro and more options for header generation

[gergondet]

This PR:

• Introduce and document PROJECT_CUSTOM_HEADER_DIR instead of CUSTOM_HEADER_DIR
• Add options to skip the generation (and thus installation) of some headers
• Add the @PACKAGE_CPPNAME@_DEPRECATED_HEADER macro which is a slightly easier to use version of the macro in warning.hh largely inspired by https://stackoverflow.com/questions/68306977/how-can-i-deprecate-a-c-header