gnulib-tool: fix imprecise comments w.r.t. an automake bug
It's not just Automake versions < 1.9b that creates an empty
pkgdatadir at installation time if pkgdata_DATA is specified
to empty; modern automake versions do this as well, at least
until automake 1.11.4 (not yet released at the moment of writing,
but soon to appear). That behaviour was generally considered a
feature rather than a bug, at least until this discussion:
<http://lists.gnu.org/archive/html/automake/2012-03/msg00014.html>