respect LIB_INSTALL_DIR and LIB_SUFFIX variables

One can change install paths by setting them during configure step. I.e.
`cmake -DLIB_INSTALL_DIR=/mars/attacks` .. and then `make install` will
install library to /mars/attacks/vdpau. LIB_SUFFIX is a "light version",
its value added to `lib` if LIB_INSTALL_DIR is not set.