cmake: use CMAKE_INSTALL_LIBDIR instead of hardcoding "lib", etc. These came from HaikuPorts, specifically this patchset: https://github.com/haikuports/haikuports/blob/462947dd4f285a885d8011cade7cd0f7151cfe74/media-libs/libsdl2/patches/libsdl2-2.0.14.patchset This is just the part that is _not_ Haiku-specific. I wanted this in a separate commit though, since I'm not really a CMake expert; if this causes problems and we need to bisect to it, we won't be confused about it being a Haiku issue when we get here. Fixes #4092. (and probably others.)