windows (mingw-w64): name of import lib

[vtorri]

on Windows, with mingw-w64, usually the extension of the import lib is not .lib but .dll.a (see https://github.com/facebook/zstd/blob/dev/lib/Makefile#L164 )

I can provide a PR if you want

[terrelln]

That would be great!

[vtorri]

see #1889