use CMAKE_INSTALL_DOCDIR insted of ${CMAKE_INSTALL_DATAROOTDIR}/doc/ccd

[expeditioneer]

Could you please use CMAKE_INSTALL_DOCDIR instead of "${CMAKE_INSTALL_DATAROOTDIR}/doc/ccd" for documentation installation? This makes it easier to change the location when an other location is desired, e.g. in Gentoo.

See CMake documentation

[RolandHughes]

A problem with CMAKE_INSTALL_DOCDIR is that on some platforms; notably Windows 10 command line building with Visual Studio compiler in the x64 Native Tools Command Prompt; it has no default value.