Use CMAKE_INSTALL_* variables provided by GNUInstallDirs.

The paths are hard-coded relative to ${prefix} which results in shared libraries getting installed in usr/lib/ instead of the  multiarch usr/lib/<arch> which poses a problem for the Debian package. This patch adds an include on GNUInstallDirs.cmake and replaces the hard-coded variables with their CMAKE_INSTALL_* equivalents, enabling a cleaner handling of installation DESTINATIONs.