gnulib-tool: fix imprecise comments w.r.t. an automake bug

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>

See also automake bugs #10997 and #11030.

* gnulib-tool (func_emit_lib_Makefile_am): Adjust comments.  Add
reference to relevant automake bug numbers.
(func_emit_tests_Makefile_am): Likewise.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>