Ubuntu/Debian introduced "multiarch" library directories, which is
unsupported by CMake <= 2.8.10. Add manual search paths as a workaround.
Currently it is only needed for libsvn, but it is needed for any lib
which is converted to the new directory standard.
Keep distro packagers happy by having the option to use the system's sqlite library instead of our bundled version. Set SYSTEM_SQLITE=1 when running cmake to use the system one.