Tex: re-add .aux target to makefile
The target to produce an intermediate .aux file for latex documents was removed in 071d7b5c to simplify the build without taking the overall docs/ makefile into account. The latter relies on a destinct target for .aux files. This commit adds it back but makes sure that failed attempts to build it still get retried by using pdflatex' drafmode (which does not write the PDF file itself). Also, fix some other minor issues in the makefile and the generation of the directory of acronyms.