ceur-ws/paper-01.pdf should depend on creation of ceur-ws directory

[clange]

The Makefile rule

ceur-ws/paper-01.pdf: ceur-ws ID

is re-executed whenever ceur-ws is newer than ceur-ws/paper-01.pdf. The timestamp of the directory ceur-ws gets updated whenever a directory entry is added/deleted/renamed.

But we mean that before creating ceur-ws/paper-01.pdf, the directory ceur-ws should be created.

This could be controlled by creating, in the same rule that creates the directory, a hidden file ceur-ws/.directory, and depending on that file. However this file would have to be excluded from the ZIP.