Make help-extract.pl work with Perl 5.10.x.

Make help-extract.pl work with Perl 5.10.x.

The main issue was use of s///r; the /r modifier was added in 5.14.
Since the baseline is now 5.10.0 instead of 5.6.0, a couple places
can be tidied up using slightly newer constructs.

* help-extract.pl: Use File::Spec::Functions instead of File::Spec
  for reduced clunkiness.
  (eval_qq_no_interpolation): Don’t use s///r.  Do use \K and (?=...)
  to match the empty string (but only in just the right locations),
  so we don’t need to use $& in the replacement.
  (main): Don’t use s///r.