Make t/xs.t et al pluggable
Refactor dynamic_lib
Refactor dlsyms
Refactor dynamic_bs
Refactor xs_o and xs_c
Test, fix static build XS
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 t/xs.t subdirsstatic to test.
unify static library linking
PASTHRU wasn't tested, didn't work. Now is, does.