bisect-runner.pl must patch Makefile.SH to avoid parallel make problems.

bisect-runner.pl must patch Makefile.SH to avoid parallel make problems.

Patch in all 4 "extra_dep" rules for XS modules if any are needed as it
simplifies the implementation. It does no harm to have dependency rules for
XS modules "from the future", as they are ignored if the module is not
present. None were needed before Cwd was first converted to an XS module,
so use that as the test for applicability.

Remove a short-lived set of Makefile rules that attempted to run the regen
scripts if needed (commits 9fec149bb652b6e9 and 5bab1179608f81d8), as they
obscure whether correctly regenerated headers were checked in, and can cause
spurious rebuilds or timing-related parallel make failures.

Remove the code to explicitly set @INC in POSIX's Makefile.PL, as the @INC
it sets will cause build failures with make_ext.pl if Cwd isn't built first,
whereas the @INC set by make_ext.pl has no such issue.