Improve handling of import code from library

On Windows:

• Items starting with -l are ignored
• Everything else is treated as an ASCII library file

On other platforms:

• Items starting with -l are treated as shared libraries
• Items containing a dot are treated as normal files and linked
• Other items are ignored

This allows one to use for example:

import code from library "mylib_library"
import code from library "-lmylib"

(where Clean System Files\mylib_library refers to mylib.dll or so). The first entry is used on Windows; the second on non-Windows.