In particular remove the ability to expel incomplete features from the
library at configure-time, since this can now be done through the
-funstable compiler flag.
Also, never provide symbols related to incomplete features inside the
dylib, instead provide them in c++experimental.a (this changes the
symbols list, but not for any configuration that should have shipped).
Is this change really needed, should we instead set -fexperimental-libray unconditionally.
Drive-by why -lbenchmark instead of benchmark?