cmake/platform/unix: add boost-1.53 no scoped enum definition

this will fix every linking error caused by using API with a boost emulated scoped enum emulation under boost-1.53 (currently only boost::filesystem::copy_file).

We have to forcing scoped enum emulation, because distributions will compile boost against the c++03 ABI and we are using the c++11 ABI. On the first one, boost is emulation scoped enums with namespaces on the latter it will use the natives one resulting into incompatible APIs.

fixes #533

NOTE: this is a candidate for the stable branch