Most icons themes just copy a bunch of files from the source to the
installation directory and therefore work perfectly fine with
stdenvNoCC.
All themes in pkgs/data/icons that still use stdenv after this change
are failing to build with stdenvNoCC.
This fixes issue where each theme installed the entire
build directory, not just the final generated 'out/X11' files.
(which was wasteful but also meant they didn't work
as the right files were two directories deeper)
Avoiding the w32 saves some time and sapce,
and more is "saved" by not shipping non-cursor build intermediates.
Sorry for the bad update, folks!