partway through implementing OPA using C11
finish off impl of existing features; removed emulation.  need to add missing stuff.
implement the rest of the operations found in opa_unsafe.h
choose C11 atomics before anything else (besides unsafe)
add some configure magic for C11 impl
fix a pass-by-val/ref bug and add a bunch of explicit casts for void* <-> intptr_t
remove incorrect casting
add FIXME related to CAS
resolve issues with CAS return value by resorting to not-actually-atomic impl
learned about returning old value in expected but not able to get correct results yet
add c11 test to automake
add c11 test to git
fix cas impl.  expected holds old val either way.