build: override bundled libsemigroups install paths

[jengelh]

Fixes: #1073

[james-d-mitchell]

Thanks for the PR @jengelh, the "with-external-libsemigroups" is expected to fail at the moment, but not the Cygwin job, any chance you could adapt the changes in this PR so that the Cygwin job also passes? Thanks!

[jengelh]

cygwin x64

Now running tests from tst/teststandard.g
Error, Variable: 'SemigroupsTestInstall' must have a value
Error: Process completed with exit code 1.

That does not look like dll related things (or if, it's not very indicative).

[james-d-mitchell]

It states just above this that the kernel module isn't compiled, I guess it is unable to find the DLL, which would be related to your changes I think