Merge ELPA changes fixing some byte-compile warnings.

* style/siunitx.el (tex): Require tex because its indispensable
when compiling the call to TeX-auto-add-type.

* style/acronym.el (tex): Ditto.

* style/acro.el (tex): Ditto.

Signed-off-by: Tassilo Horn <tsdh@gnu.org>
Check the viewer executable exists before opening it.

* tex.el (TeX-view-program-list-builtin): Add the new third
element to each item.
(TeX-view-program-list): Document new third element of items in
doc string.  Adapt the type.
(TeX-view-command-raw): Check whether the viewer executable
exists.  (Bug#19121)

* doc/auctex.texi (Starting Viewers): Document change to
`TeX-view-program-list'.

* doc/changes.texi: Mention it.