Incompatibility with `libsemigroups` v3

[james-d-mitchell]

I'm creating this issue to raise visibility of the fact that the current version of Semigroups requires libsemigroups v2 and is incompatible with libsemigroups v3. As a consequence the configure option --with-external-libsemigroups won't work on a system where (for example) the latest version of libsemigroups is installed using conda (since there's no conda package for the latest v2 release of libsemigroups, and if you just conda install libsemigroups or equivalent, then you get v3).

Here's a list of related issues:

• The semigroups package can't be loaded: it builds libsemigroups.so.2 but gap looks for libsemigroups.so.3 during the run-time gap-system/gap#6102