runtime: Use "/usr/share" as a fallback lib data directory
If the host system stores the shared libraries in a directory that is
not what we expect, we try as a last resort /usr/share
, in an effort
to increase the number of OSs that we are able to cover.
For example this should help Exherbo to find the right libdrm and drirc.d directories.
Fixes: #68 (closed)