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>