Static desktop-file-validate and zsyncmake

[probonopd]

Build static desktop-file-validate and zsyncmake similar to what we are already doing for mksquashfs.

Closes #9

[probonopd]

$ ./appimagetool-x86_64.AppImage --appimage-extract
$ file squashfs-root/usr/bin/desktop-file-validate
squashfs-root/usr/bin/desktop-file-validate: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, BuildID[sha1]=..., stripped

[Samueru-sama]

Please merge this. I'm building appimages on alpine and they don't have zsync on their repo.

Btw @probonopd I tried to use your static zsyncmake and it isn't working, even though appimagetool says that everything went right with zsyncmake no .zsync file ever gets made.

[probonopd]

@TheAssassin ready for merging?

[TheAssassin]

This PR falls short of some expectations. You still need file to be installed on the system. appstream-util is another optional dependency.

[probonopd]

There is no way for the user to use a newer release or custom binary of desktop-file-validate. This means that in theory, you need to keep this tool very much up to date to fix potential compatibility problems with newer versions of the specification.

@TheAssassin feel free to add a Dependabot in a separate PR.