Fix build when Qt is used in an automatic reference-counted environment

When automatic reference-counting (ARC) is enabled on Darwin platforms
the NSAutoReleasePool class should not be used directly, which caused a
build error if qglobal.h was included after the Foundation.h in client
code.

The preferred alternative for ARC is the scoped @autoreleasepool
construct, which allows the compiler to reason about needing to
insert _objc_autoreleasePoolPush and _objc_autoreleasePoolPop
calls.

Note that ARC translation units can be combined with non-ARC translation
units, so Qt and the QMacAutoReleasePool class can still be used in
ARC client code even if Qt is not built with ARC.

Task-number: QTBUG-51332
Change-Id: I7ef1c3146aa416a9d6a1dc299ce7b17f22f889e5
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@theqtcompany.com>

src/corelib/global/qglobal.h

@@ -579,10 +579,6 @@ Q_DECL_CONSTEXPR inline const T &qBound(const T &min, const T &val, const T &max
 #  define QT_OSX_DEPLOYMENT_TARGET_BELOW(osx) \
       QT_MAC_DEPLOYMENT_TARGET_BELOW(osx, __IPHONE_NA)
 
-QT_END_NAMESPACE
-Q_FORWARD_DECLARE_OBJC_CLASS(NSAutoreleasePool);
-QT_BEGIN_NAMESPACE
-
 // Implemented in qcore_mac_objc.mm
 class Q_CORE_EXPORT QMacAutoReleasePool
 {
@@ -591,7 +587,7 @@ public:
     ~QMacAutoReleasePool();
 private:
     Q_DISABLE_COPY(QMacAutoReleasePool)
-    NSAutoreleasePool *pool;
+    void *pool;
 };
 
 #endif // Q_OS_MAC

src/corelib/kernel/qcore_mac_objc.mm

@@ -150,7 +150,7 @@ QMacAutoReleasePool::~QMacAutoReleasePool()
     // Drain behaves the same as release, with the advantage that
     // if we're ever used in a garbage-collected environment, the
     // drain acts as a hint to the garbage collector to collect.
-    [pool drain];
+    [static_cast<NSAutoreleasePool*>(pool) drain];
 }
 
 // -------------------------------------------------------------------------