Make hstore_plperl's build more like plperl's

Make hstore_plperl's build more like plperl's

This involves moving perl's CORE library to the end of the include list,
and adding other compilation settings that plperl uses. This won't
completely fix the breakage currently being seen by gcc builds on
Windows, but it will let the build get further, and should be wholly
benign, if not beneficial, on *nix.