[exiv2] enable MinGW support
Related: !250 (closed) and !190 (merged)
exiv2 0.27.5 should now also be able to compile with MinGW. !190 (merged) did cleaned up the old revisions but leaves MinGW marked as unsupported, since it's now able to build 0.27.5 under MinGW, I guess we could mark it as supported again :)