Remove constructor version attribute on OS X

Remove constructor version attribute on OS X

Summary:
OS X doesn't support this -- it requires support at the linker
level which OS X just doesn't have. Reasonable OS X compilers will yell
about it, but even the ones that don't are just silently dropping it
on the floor.

So I'm not sure what //we// can do other than just silently dropping
this on the floor too and hoping that everything works.

Test Plan:
HHVM build on OS X gets ever so slightly further (runs into
other issues). HHVM build in fbcode still seems to build folly okay
(didn't wait for it to finish fully).

Fixes https://github.com/facebook/folly/issues/78

Reviewed By: tudorb@fb.com

FB internal diff: D1487191

Blame Revision: rFBCODE502be7cb0fda306ddc33b9c5ad7a53b0cfbdc893