Fix doc installation

CMake doesn't understand the bash syntax to list several files in the same dir it seems, so I'm using another method :-)

The previous implementation gave this error:
> CMake Error at cmake_install.cmake:122 (file):
>  file INSTALL cannot find
>  "/home/iurt/rpmbuild/BUILD/opendungeons-0.4.9-rc1/{AUTHORS,CREDITS,FAQ.txt}".