Linker warnings

[james-d-mitchell]

I get the following warning from the linker:

*** Warning: Linking the shared library NautyTracesInterface.la against the non-libtool
*** objects  /Users/jdm/gap/pkg/NautyTracesInterface/nauty26r7/nauty.o /Users/jdm/gap/pkg/NautyTracesInterface/nauty26r7/nautil.o /Users/jdm/gap/pkg/NautyTracesInterface/nauty26r7/naugraph.o /Users/jdm/gap/pkg/NautyTracesInterface/nauty26r7/schreier.o /Users/jdm/gap/pkg/NautyTracesInterface/nauty26r7/naurng.o /Users/jdm/gap/pkg/NautyTracesInterface/nauty26r7/naugroup.o /Users/jdm/gap/pkg/NautyTracesInterface/nauty26r7/nautinv.o is not portable!

[fingolfin]

This is harmless in so far as it works on Linux and macOS, and we are not really interested in other operating systems.

There could be an issue in settings where there are other libraries loaded in the process which also try to link against some copy of nauty; in such situations it is better to link everything against a single copy of nauty, which then should be in a shared library. But that's what --with-nauty=<path> is for (at least in theory, see issue #23)