issue121: refactor -DALLOW_PERL_OPTIONS tests and simplify logic

on threaded perls -s is not processed, -e "" is not appearing in S_parse_body in pureperl
C major change: skip redefinition of internal XS CVs

Internal XS xsubs are already booted in the perl_parse step.
When we encounter them we set new GPs for those, which was a problem on Windows
where we cannot declare and boot the said XS functions (not exported).
We rather keep the old GV, GP and CV.
Merge branch 'issue121' into release

Conflicts:
	Changes