Fix build on OS X using libc++ (there's currently an ambiguity bug when using std::async, but it will be gone in the next release,  hence all the ugly conditional compilation).
Fix build on OS X using libc++ (there's currently an ambiguity bug when using
std::async, but it will be gone in the next release,  hence all the ugly
conditional compilation).

seec-team/seec,mheinsen/seec,mheinsen/seec,seec-team/seec,seec-team/seec,seec-team/seec,mheinsen/seec,seec-team/seec,mheinsen/seec,mheinsen/seec