The typical situation where the usual, plain 'lib' is not wanted is in a
multiarch system, that allows installation of the same library built for
multiple architectures.
Use the standard ${CMAKE_INSTALL_LIBDIR} variable (coming from the
GNUInstallDirs module) to figure where to put the built libraries.
Signed-off-by: Mattia Rizzolo <mattia@mapreri.org>