Respect -DLIB_SUFFIX option to support proper "make install" on systems with lib64

Some Linux distributions use /usr/lib64 instead of /usr/lib for 64-bit libraries. It includes distros like Slackware, AgiliaLinux, most Gentoo installlations and some others.

A de-facto common way to build CMake-based source in these cases is to provide -DLIB_SUFFIX=64 to cmake options. For example, this way is used in KDE sources.

This patch implements support for this option.