gh-36219: configure.ac: disable webencodings,tinycss2 with --disable-notebook
The `--disable-notebook` flag disables the bleach and nbconvert
packages, and the webencodings and tinycss2 packages are (solely)
dependencies of those. We can therefore disable webencodings and
tinycss2 whenever `--disable-notebook` is given.
URL: https://github.com/sagemath/sage/pull/36219
Reported by: Michael Orlitzky
Reviewer(s): Matthias Köppe
