l3build: add "typesetsuppfiles" variable

it seems useful to be able to add files to "support/" that can be used in the compilation of doc.
otherwise you'd need to litter the main folder with files for typesetting.

longer term, we might want to consider having a separate build directory for typesetting in which the finished product is moved back into the main dir if desired.

other considerations: typesetruns counter, inclusion of bibtex/biber, glossaries, ... perhaps totally separate build scripts that we can just call externally? Don't want to re-invent the wheel too much in here.

git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5095 de43f980-851b-0410-b2f7-c40aca1f87e0