Pimp my makefile.am

Ok, things have worked as well as I wanted and I think we have a nice improvement here. The build system should never be a bother, and I think the simplifications I propose will really easier to maintain.

First thing I did was to remove all autogenerated files from the repo, and added them to the .gitignore. This is why there's a big quatity of line changes in the merge.

I also rewrote most of the manual installation routines into standard Makefile.am code. As far as I can tell, all lands in the right place and is much easier to read and maintain.

Additionally, the icons get installed in the right directory, with just a simple list to maintain if there are other icons to install someday either in the global /usr/share/icons directory or the pixmaps in /usr/share/siril/pixmaps

Two things bother me, but I don't think it's too big of an issue:

• I need to double check that in a majority of the distros, the icon dir is indeed /usr/share/icons
• I know you've got conditionals here and there for windows and osx, I think my changes are still compatible, but if you could test that on your side it would be cool. I don't know how you build your packages for those two systems.

I'm setting this to WIP, because I still expect a few changes either from you, or simplifications that I might still find. I'm sure I can still go an extra mile.

Oh, and I might need to rewrite my history on my forked repo. I had the email of my previous job set for commits... bummer 😄