file_ops.c: dynamically load SHGetKnownFolderPath

See issue #305 for the problem details.

This pull request contains two commits:

c821e3e special-cases mingw32 and doesn't touch msvc behavior
7112434 uses dynamic linking of the DLL function for all windows

Hopefully 7112434 does the trick and doesn't blow up Windows.  If it does, revert to c821e3e.  I do not have the capability to test this beyond mingw32 compiles.