Enable LTO build

[ashquarky]

This PR includes changes to mark the important .wups sections with the "used" attribute. This ensures that even aggressive optimisations (e.g. LTO) will not remove the hooks and WUPS metadata.

Since the used attribute needs to be on the variable itself (not an extern), move all attributes to the used attribute. extern in C++ also implies C linkage, so specify C linkage.

With that groundwork in place, enable an LTO build. This isn't super important - the used attributes are needed for plugins to use LTO - but it's basically free to enable it for the library too, so why not?

[Maschell]

How "safe" is it to use lto, i've been told
that this can be quite unstable from time to time. Maybe remove this from WUPS to be sure, i doubt it'll have any big impact on this lib anyway?

[ashquarky]

LTO is used by the PPC64 builds of Firefox - so it can at least hold its own for the most complex piece of C++ imaginable for a similar CPU arch. It used to be pretty buggy but word is that it's much improved.

Note I enabled -ffat-lto-objects - this means that if the plugin doesn't use LTO, they will be completely unaffected by this change. That flag basically runs two compiles - one normal, one LTO, and puts the results of both in the .o files. So every plugin in the wild will just ignore the LTO parts of the file and get the same -O2 code they've always had. You can confirm this with readelf, they add a bunch of new sections like .gcc_lto_main.c_...

So even if LTO on ppc32 does generate buggy code, plugin developers have to opt-in to it by passing -flto to the final, plugin link step; nobody does this so every plugin in the wild will not be affected. That's why I said it was "basically free"; for plugin devs who want to play with it.

[ashquarky]

(the use, for what it's worth, is that I'm trying to get the binary size down for Inkay as much as possible, given that it's basically a shell plugin at this point with nothing but a config menu in it. I know there are constraints on binary size that the Aroma website imposes, so I'd like to make room for actual features 💙)