The `make check` target used to have a list of targets (inkscape,
lualatex, et c.) but now loops through the variable `$(dependencies)`.

This means projects can add individual dependencies in their makefiles
like this:

`dependencies += magick`

The new dependency will be checked when running `make check`.

The books now keep common makefile commands in config/vars.  A new
command will give pdfs titles according to the first header in the
README file.

bash and dash have different outputs, which means the compilation
fails for one or the other.  By defining the QR code entirely with
makefile syntax, the QR code works on both.