Search LD_LIBRARY_PATH while looking up shared-objects

With this, we can load a `so` file not listed in `/sbin/ldconfig -p` by setting the `LD_LIBRARY_PATH`, as we what we usually do  on Linux/FreeBSD:


```
$export LD_LIBRARY_PATH=/path/to/libdira:/path/to/libdirb
$julia a.jl
```