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.

[TheAssassin]

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. Another option is to let a system-wide-installed binary precede the one contained in the AppImage (something that requires a few more lines of code and some additional directory for the $PATH) or maybe a dedicated environment variable to specify the path exactly (e.g., DESKTOP_FILE_VALIDATE_PATH=/usr/bin/desktop-file-validate).

[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.