videodb: don't reset the source path if it is temporarily offline
videodb: add GetSourcePath() to get the path of the source another path belongs to
videodb: use the proper source path when checking for a temporarily offline source during library cleaning
[win32] update readme
Merge pull request #4795 from Montellese/videodb_clean_fix

videodb: fixes for support of temporarily offline sources during library cleaning
Revert "[depends] add m4 macros for compiler flag testing"

This reverts commit 6097ea3e9437c9cdef92c2f28e8cb8710af96017.
FIX: fixup ac39d8f1e48d0a875c411607d5c8d60fbd09ca3e