Target specific architectures for OS X.

The OSX_ARCHITECTURES property sets the target binary architecture for targets on OS X (-arch). This property is initialized by the value of the variable CMAKE_OSX_ARCHITECTURES if it is set when a target is created. Use OSX_ARCHITECTURES_<CONFIG> to set the binary architectures on a per-configuration basis, where <CONFIG> is an upper-case name (e.g. OSX_ARCHITECTURES_DEBUG).