Use the CMAKE_INSTALL_LIBDIR variable instead of the hard coded "lib" directory
Fixes the following bug: https://bugs.launchpad.net/inkscape/+bug/1578164
- CMakeLists.txt: Modified file (use the CMAKE_INSTALL_LIBDIR variable instead of the hard coded "lib" directory).
- src/CMakeLists.txt: Modified file (use the CMAKE_INSTALL_LIBDIR variable instead of the hard coded "lib" directory).