Windows cross build documentation

[RikyPlaza]

Updated documentation for cross compilation of nut project with windows target.

The steps have been tested on Ubuntu 22.04 virtual machine (VirtualBox) but the binaries have not been tested yet.

[AppVeyorBot]

❌ Build nut 2.8.2.2558-master failed (commit 3ab41980d2 by @RikyPlaza)

[jimklimov]

Thanks, although I think a few typos for spellchecker to catch remain, some lines are too long, and I'm not sure I agree with libneon change.

[jimklimov]

Actually, upon review most of the suggested fixes look questionable. For example:

• Repetitive PKG_CONFIG_PATH is not needed if you re-use the same terminal where you have already set the variables according to big note in the beginning.
• Explicit installation of pkgconf is also not needed if you followed docs/config-prereqs.txt and installed the needed toolkits.
• Not sure which changes VirtualBox Guest Additions would bring that prerequisites don't already deliver. In context of font library I suppose something from X11 about graphics core?.. Notably, as you build something for another architecture, the headers, libraries and tools made for the build host itself are not all directly impactful.
• Likewise, PATH=${PREFIX}/bin:... would include the location of Windows *.exe programs that your build host is not necessarily able to execute.
• Explicit libtoolize etc. are not needed as you call the autogen.sh just afterwards, which involves them.
• The CLI options added for ci_build.sh are not supported by the script.

Maybe something else...

Still, this PR did highlight some parts that could be more understandable - so thanks! I'll cherry-pick some of the changes into a new PR and add some clarifications, but would rather close this one.

[RikyPlaza]

@jimklimov Hi!
Regarding the re-definitions of the environment variables you're totally right, i inserted them since i was not fully dedicated to this topic and i had to work in spots... In any case i don't think it's dangerous to have them
Regarding the PATH=${PREFIX}/bin:... yes, but the libtool binaries are installed in that location and would not be reachable if not exported...
the explicit libtoolize has been inserted since the autogen.sh script is was not working on my system, maybe something related to the fact that i'm using a virtual machine

using the commands reported in the docs to cross build the software caused every time some errors, this is the only way i got it working.

Great for the cherry picks! Hope that at least something in this PR can be usefuk :)

Tomorrow i'll be back in the office, hope to have chanche to test the software on windows as soon as possible.

Regards