Add JonY's buildsystem changes to support 6 warning levels, 0 through 5.

See ./configure --help for details on the new option and how it works.



git-svn-id: svn+ssh://svn.code.sf.net/p/mingw-w64/code/trunk@1177 4407c894-4637-0410-b4f5-ada5f102cad1