gh-38658: configure.ac: add --disable-sagetex option
SageTeX is a standard package, but it isn't used anywhere else in the
sage library or even in the sage distribution. If you know you don't
need it, the new `--disable-sagetex` option lets you avoid downloading
and installing it.
(inspired by https://github.com/sagemath/sage/issues/38657, but I should
have done this a long time ago)
URL: https://github.com/sagemath/sage/pull/38658
Reported by: Michael Orlitzky
Reviewer(s): Dima Pasechnik
