Make sure INTEFACE-declaration works for C, too.  Additional reviewed interface-layout and removed warnings for verified ones.


git-svn-id: https://svn.code.sf.net/p/mingw-w64/code@6496 4407c894-4637-0410-b4f5-ada5f102cad1
Verify Interfaces and make them C-compatible


git-svn-id: https://svn.code.sf.net/p/mingw-w64/code@6497 4407c894-4637-0410-b4f5-ada5f102cad1