all: remove use of 'register' keyword

We should have done this a decade ago...

https://bugzilla.gnome.org/show_bug.cgi?id=730293
gmessages: make g_assert_warning compiler friendly

https://bugzilla.gnome.org/show_bug.cgi?id=720708
Convert multiple #if/#endif to single #if/#elif/#endif

https://bugzilla.gnome.org/show_bug.cgi?id=731424