Replace ${CMAKE_INSTALL_PREFIX}/share with DATAROOTDIR

Summary:
This allows to specify the DATAROOTDIR according to GNUInstallDirs.
It is important for distributions like Exherbo that install arch-dependent
files to /usr/${host} and arch-independent files to /usr/share

Test Plan: Builds and installs the files to the correct locations.

Reviewers: Marble

Projects: Marble

Differential Revision: https://phabricator.kde.org/D768