Several incompatibilities with RubyInstaller-2.4

[larskanis]

The ruby-filemagic gem has several incompatibilities with RubyInstaller-2.4.

1. The ruby_filemagic.so load error in the binary gem can probably be fixed per RubyInstaller::Runtime.add_dll_directory - see here.
2. The gem should add a msys2 library dependency to the mingw-w64-file package to support native builds. I already opened a PR to make it available in the MSYS2 repo.
3. The gem should not search in /usr/include or /usr/lib on MINGW. These directories contain CYGWIN/MSYS2 related files, which can not be used together with RubyInstaller. For RubyInstaller-2.4 it's enough to use the predefined library paths, provided the file package is installed. dir_config is usually not necessary.
4. As a recommendation: rake-compiler-dock is maybe a more standardized way to build binary Windows gems. It can replace the project specific Dockerfile and docker handling.

This came up due to the following issue on RubyInstaller: oneclick/rubyinstaller2#64

Yep, got bitten by this today too.

[blackwinter]

I'm sorry but I'm no longer using this library nor RubyInstaller/Windows. Hence, I've decided to give up maintainership. Please contact me if somebody wants to take over.