add support for LD_LIBRARY_PATH env var

[roomsg]

It would be very convenient to have more control on the paths (and order) queried for loading native libraries (apart from the std three currently taken into account); prepending the paths from LD_LIBRARY_PATH evn var would be a very welcome addition and common linux behavior.

The underlying jnr-ffi lib does allow to append some extra paths, but unfortunately those did not help us out (ref jnr/jnr-ffi#129). They were only appended to the std search paths.