CMake: Avoid hard-wired /usr/local/share

[gollux]

Instead of defining own CMAKE_INSTALL_SHAREDIR, use existing CMAKE_INSTALL_FULL_DATADIR.

This way, CMAKE_INSTALL_PREFIX is respected.

[gszy]

Looks OK except an indentation issue.

[Nightwalker-87]

@gollux: Thanks for your contribution. The change looks fine to me.
However we should add a note in our tutorial on how to select a custom install path (e.g. link to https://cmake.org/cmake/help/latest/variable/CMAKE_INSTALL_PREFIX.html) to point users to it, in case they are not familiar with CMake.

[Nightwalker-87]

I'll update the documentation afterwards.