This avoids generating the tags files all the time, and makes `make
install` with `CMAKE_INSTALL_MESSAGE=LAZY` much more silent in general.
Using `copy_if_different` instead of `remove` + `copy_directory` might
be good on top, but is a) not really necessary anymore and b) would not
sync removed files.
For this `file(COPY` could be used, but would require to re-run cmake on
changed input files then.