Change the operating system name from OSX to MacOS everywhere

Summary:
- Changed the name in all build artifacts
- Everywhere in C++

Reviewers: #swift_pilot_client, kbasan

Reviewed By: #swift_pilot_client, kbasan

Subscribers: jenkins

Maniphest Tasks: T205

Differential Revision: https://dev.swift-project.org/D69

src/blackgui/windowfinder.cpp

@@ -14,7 +14,7 @@
 #if defined(Q_OS_WIN)
     #include "win/windowfinderwindows.h"
 #elif defined(Q_OS_LINUX)
-#elif defined(Q_OS_OSX)
+#elif defined(Q_OS_MACOS)
 #else
     #error "Platform is not supported!"
 #endif
@@ -28,7 +28,7 @@ namespace BlackGui
 #if defined(Q_OS_WIN)
         finder = new CWindowFinderWindows();
 #elif defined(Q_OS_LINUX)
-#elif defined(Q_OS_OSX)
+#elif defined(Q_OS_MACOS)
 #endif
 
         return finder;