Fix REGEXP support on 5.17.6 and up

With commit 8d919b0a35f2b57a6bed2f8355b25b19ac5ad0c5 (perl.git) and
release 5.17.6, regular expression are no longer SvPOK (IOW are no longer
considered to be containing a string).
This breaks some of the REGEXP detection logic in srl_dump_sv, so
we need yet another CPP define.