CMake modules

The engine’s CMake surface, extracted from #[==[.rst: bracket comments in the .cmake sources themselves (D3) — not hand-written here.

• CMake modules
• common.cmake
• doxygen.cmake
• sphinx.cmake
• registry.cmake
• zdocs.cmake
• download_external_tag.cmake
• run_doxygen.cmake
• check_doxygen_warnings.cmake