Create libmysql.a using dlltool.exe on mingw64

~~I have no idea if this will work.~~ It works! See #469.
`libmysql.def` is checked into the repo, because tools to generate it at compile time are not a default part of MinGW :(
`libmysql.lib` is referenced from the MySQL directory, since it is only needed a compile time.
`libmysql.dll` is copied over because it is needed at run time.