Support for OSX

Implements #364 (closed). We:

• Detect whether the POSIX library implements the barriers or not.
• If they are implemented define the SWIFT barriers as the POSIX ones.
• If not use an alternative, simple, implementation.
• Be verbose about the implementation being used.
• Update the autotools macro that sets pthread flags to the latest version. Should handle OSX in a better way.
• Detect whether FPEs can be raised on this system and set the appropriate macro.
• Update the INSTALL.swift with instructions for OSX.
• Detect Skylake (mobile and desktop) and set the appropriate architecture flags.
• Fix the dump and logger tests to write unique files to /tmp/ and delete them once done.