Since CMake 3.11, the supported method of integrating Doxygen into
CMake projects has been via the doxygen_add_docs() CMake function,
which (crucially) can be passed a target name to create and doesn't
rely on tons of hardcoding like our UseDoxygen.cmake and its
Doxyfile.in template did.
This means our docs (only) now require CMake 3.11 to generate, but it
allows Doxygen to be configured/used multiple times in the same CMake
configuration, removing a major barrier to superproject builds.
Also, made docs disableable via an ENABLE_AUDIO_DOCS option, and adopt
FeatureSummary into the build.
Remove Doxyfile.in template
CI: Don't autoinstall API docs
Delete Doxyfile.in
Docs: Use PROJECT_VERSION_FULL
Don't pre-process source files
There was a bug in the definition of `DOXYFILE_LATEX`, and _another_
in the processing of it (which were the only reason it wasn't
defaulting on, the way it appeared to be configured), fixed the
bug and changed it to default OFF. But now it _can_ be enabled.
Also moved the handling of `DOXYFILE_DOT` out of the latex-only
section, so that it can be turned on with the new config variable
`DOXYFILE_USE_DOT` (default on). If `DOXYFILE_USE_DOT` is enabled
and the `dot` executable is found, `DOXYFILE_DOT` will be set "YES"
and `dot` will be used for the HTML as well, giving better graphs.
