Break pure_all target into pure_nolink and linkext.
This allows breaking subdirs into subdirs_static and subdirs_dynamic.
This in turn allows building either static or dynamic when subdirs.
Add test.
unify static library linking
PASTHRU wasn't tested, didn't work. Now is, does.
Make $DynaLoader::bscode in *_BS get written to *.bs even if no @all
Test, fix Mkbootstrap functionality - fix RT 6176