lib2geom uses `HAVE_GSL` which is never set in Inkscape builds
I just noticed 2geom guards multiple sections of code with #ifdef HAVE_GSL
.
However - despite Inkscape depending on GSL - the build system does not seem to set HAVE_GSL
anywhere.
It seems to have worked just fine so far (so this is probably not a high priority) however we should likely try to figure it out eventually (unless we choose to delay it until we switch to an external copy of lib2geom).