Fix breaking GNUInstallDirs cmake module $CMAKE_INSTALL_LIBDIR directory. Fix #4621 According to https://cmake.org/cmake/help/v3.0/module/GNUInstallDirs.html `${CMAKE_INSTALL_LIBDIR} ` is only variable which should be used as base directory. Without that patch on 64 bit archs libraries, cmake modules and pkgconfig file are installed for example in /usr/lib6464 base directory. This patch fixes #4621. Signed-off-by: Tomasz Kłoczko <koczek@github.com>