automake: new variable AM_DISTCHECK_DVI_TARGET to override "dvi".

automake: new variable AM_DISTCHECK_DVI_TARGET to override "dvi".

This change fixes https://bugs.gnu.org/8289.

* lib/am/distdir.am (AM_DISTCHECK_DVI_TARGET): define as dvi.
(distcheck): use it, isntead of hardcoding dvi.
* lib/Automake/Variable.pm (%_silent_variable_override): add
AM_DISTCHECK_DVI_TARGET.
* t/distcheck-override-dvi.sh: new test.
* t/list-of-tests.mk (handwritten_TESTS): add it.
* doc/automake.texi (Checking the Distribution): document this.
(Third-Party Makefiles): explicitly mention that
EMPTY_AUTOMAKE_TARGETS is not a built-in or special name.
Various other index entries and wording tweaks.
* NEWS (Distribution): mention this.