Documentation/Makefile: make doc.dep dependencies a variable again

Documentation/Makefile: make doc.dep dependencies a variable again

Re-introduce a variable to declare what *.txt files need to be
considered for the purposes of scouring files to generate a dependency
graph of includes.

When doc.dep was introduced in a5ae8e64cf (Fix documentation
dependency generation., 2005-11-07) we had such a variable called
TEXTFILES, but it was refactored away just a few commits after that in
fb612d54c1 (Documentation: fix dependency generation.,
2005-11-07). I'm planning to add more wildcards here, so let's bring
it back.

I'm not calling it TEXTFILES because we e.g. don't consider
Documentation/technical/*.txt when generating the graph (they don't
use includes). Let's instead call it DOC_DEP_TXT.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>