cython needs to be a configure output

[baallan]

sos configure fails on many versions of linux (e.g. centos, rocky) where the functional version of cython includes as suffix. (e.g. cython-3.11, cython3). As with python in configure.ac, sos configure.ac should support a CYTHON=xxx variable and the makefiles should use $(CYTHON).

[baallan]

This is also failing on redhat8 where we are trying to upgrade production servers to current sos and 3.12 python/cython.