Add .editorconfig.
Merge pull request #987 from larskanis/verbose-clean

Do the clean after build more verbosely, so that it's logged which directories were removed.
Improve -llzma and -lexslt tweaks. #988
Add support for cross compiling a multi platform (x64-mingw32 and
x86-mingw32) fat binary gem and support for native builds on Windows.

This moves all MiniPortile recipes into extconf.rb. This enables
native build on Windows and ensures that all depending libraries
are build with the same compiler version as the C-ext when cross compiling.