Document how to cross compile about owlkettle

[can-lehmann]

I use Fedora myself and found cross compiling for Windows quite a pleasant experience. I do not currently have a Windows installation, so I could only test it using wine, but it seemed to work nicely (except for a few missing icons):

sudo dnf install mingw32-gtk3
sudo dnf install mingw64-gtk3
nim compile -d:mingw todo.nim 
sudo dnf install wine
WINEPATH=/usr/x86_64-w64-mingw32/sys-root/mingw/bin/ wine todo.exe 

Not having the mingw64-gtk3 package might complicate things somewhat (as you have described). As I do not use a debian based system, I don't think that I can be of much help here. I would however welcome a pull request adding a document describing how it can be done, should you get it to work. I do however think that dynamic linking is the way to go here, primarily for licensing reasons related to the LGPL.

In general GTK (and therefore owlkettle) might not be the best choice for Windows/Cross-Platform development anyway, since (as far as I know), GTK does not adapt its theme correctly to the operating system and therefore looks somewhat out of place. If you still really wanted to target Windows using owlkettle, a better option might be to create a second widgets.nim file which targets a different backend (instead of GTK). Large parts of owlkettle are not inherently GTK specific and could therefore theoretically be reused. This is of course a lot of work, and currently not one of my personal interests/priorities.

TLDR: Even though cross-compiling owlkettle apps to Windows is not one of my personal aims, if you find a way to get it to work properly, I would accept a pull request which adds a docs/cross_compiling.md describing the process for Ubuntu/Debian and Fedora.