cmake: make GetGitRevisionDescription work

[EchterAgo]

The current build does only update zfs_gitrev.h on the first configuration run but then not change it when the commit changes.

To fix this we need to call get_git_head_revision.

This also updates GetGitRevisionDescription to the latest version from https://github.com/rpavlik/cmake-modules/tree/d66fc5dec3eaea3a4f9778ceeeb65cbec490b5b9

[EchterAgo]

Note that we don't need to update GetGitRevisionDescription to the latest version, I just included it because I did it already.

[lundman]

I'll try it first, I wouldn't want it to slow down my compiles, since I only need it to update when I want to (saving CMakelists) :)

[EchterAgo]

Yea, I do think it slows down things because it recompiles a lot everytime the git revision changes. Also it does not update when the describe output changes, for example when going from dirty to not dirty.

Maybe I'm spoiled because it compiles really fast on my machine, but I do like having it, maybe we could make it optional?

[EchterAgo]

It is unfortunate that ZFS_META_ALIAS is defined in zfs_config.h. Not many places use ZFS_META_ALIAS but zfs_config.h is included everywhere :\

[lundman]

OK this doesnt slow down my build at all.
zfs_config.h is "ours" (they generate theirs from autotools) so if you want to move the macro that is fine.

[lundman]

I do have to comment this out now and then, whenever I do a git cherry-pick it will redo cmake and recompile everything - which is rather frustrating, when I have 100-200 commits to go through :)