avoid policy warning with recent CMake versions

See `cmake --help-policy CMP0026` with CMake ≥ 3.0.
install man page even if BUILD_DOCS isn't set

The man page is useful and does not need additional software to be installed.
clean up installation of doc files
reduce number of warning messages when building docs

Warnings were emitted for every info file (twice) instead of once.
Merge pull request #321 from kljohann/cmake

Clean up CMake files