Fix for timeval on OSX.
Fix missing header on OSX.
Fix pnUUID linking on OSX. Closes #451.
Rename UniChar to plUniChar to avoid conflict on OSX.

OSX defines a UniChar type as part of the OS headers.
Add HS_BUILD_FOR_OSX definition.
Merge pull request #454 from dpogue/osx-fixes

OSX Clang fixes