Fix local installation of plugins

Let me explain what I did:

It doesn't make sense to add CMAKE_INSTALL_PREFIX before the installation path in the path that's being used for installation because CMake will install to the prefix anyway. 
There's a reason to not do it: If the CMAKE_INSTALL_LIBDIR is already absolute, things break horribly.
One could use CMAKE_INSTALL_FULL_LIBDIR but there's really no use in doing that.

On the other hand that var is handy to be added as a defintion because it comes from the GnuInstallDirs and the CMAKE_INSTALL_PREFIX is only prepended if the path is not yet absolute.