Add Q_OS_OSX and update the documentation to reflect usage intent.

Change-Id: I7f089109d7357995b65f6a012f705a6c3cac12ff
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>

src/corelib/global/qglobal.cpp

@@ -934,7 +934,7 @@ bool qSharedBuild() Q_DECL_NOTHROW
     \endlist
 
     Some constants are defined only on certain platforms. You can use
-    the preprocessor symbols Q_OS_WIN and Q_OS_MACX to test that
+    the preprocessor symbols Q_OS_WIN and Q_OS_OSX to test that
     the application is compiled under Windows or OS X.
 
     \sa QLibraryInfo
@@ -1077,18 +1077,20 @@ bool qSharedBuild() Q_DECL_NOTHROW
     \macro Q_OS_DARWIN
     \relates <QtGlobal>
 
-    Defined on Darwin OS (synonym for Q_OS_MAC).
+    Defined on Darwin-based operating systems such as OS X and iOS,
+    including any open source version(s) of Darwin.
 */
 
 /*!
     \macro Q_OS_MAC
     \relates <QtGlobal>
 
-    Defined on OS X and iOS (synonym for Q_OS_DARWIN).
+    Defined on Darwin-based operating systems distributed by Apple, which
+    currently includes OS X and iOS, but not the open source version.
  */
 
 /*!
-    \macro Q_OS_MACX
+    \macro Q_OS_OSX
     \relates <QtGlobal>
 
     Defined on OS X.

src/corelib/global/qsystemdetection.h

@@ -49,9 +49,9 @@
 /*
    The operating system, must be one of: (Q_OS_x)
 
-     DARWIN   - Darwin OS (synonym for Q_OS_MAC)
-     MAC      - OS X or iOS (synonym for Q_OS_DARWIN)
-     MACX     - OS X
+     DARWIN   - Any Darwin system
+     MAC      - OS X and iOS
+     OSX      - OS X
      IOS      - iOS
      MSDOS    - MS-DOS and Windows
      OS2      - OS/2
@@ -175,8 +175,9 @@
 #  include <TargetConditionals.h>
 #  if defined(TARGET_OS_IPHONE) && TARGET_OS_IPHONE
 #     define Q_OS_IOS
-#  else
-#     define Q_OS_MACX
+#  elif defined(TARGET_OS_MAC) && TARGET_OS_MAC
+#     define Q_OS_OSX
+#     define Q_OS_MACX // compatibility synonym
 #  endif
 #endif