Declare Cocoa conversion funcs in objc-mode only.

In practice, there are several ways to forward-declare
objective-c classes. Qt uses "struct objc_object",
other projects may use a plain "class". Mismatched
forward declarations will lead to compile errors,
and this is a form of header pollution.

dd5e40d9 added a workaround where Q_FORWARD_DECLARE_OBJC_CLASS
can be predefined in order to sync up the declarations.

Make forward declaration clashes less likely by
forward-declaring in objc-mode only.

Change-Id: I9f7a399d64dc88bfe05d5385b3d46b5302112aef
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>

src/corelib/io/qurl.h

@@ -51,8 +51,10 @@
 #include <QtCore/qglobal.h>
 
 #ifdef Q_OS_MAC
-Q_FORWARD_DECLARE_OBJC_CLASS(NSURL);
 Q_FORWARD_DECLARE_CF_TYPE(CFURL);
+#  ifdef __OBJC__
+Q_FORWARD_DECLARE_OBJC_CLASS(NSURL);
+#  endif
 #endif
 
 QT_BEGIN_NAMESPACE
@@ -265,8 +267,10 @@ public:
 #if defined(Q_OS_MAC) || defined(Q_QDOC)
     static QUrl fromCFURL(CFURLRef url);
     CFURLRef toCFURL() const Q_DECL_CF_RETURNS_RETAINED;
+#  if defined(__OBJC__) || defined(Q_QDOC)
     static QUrl fromNSURL(const NSURL *url);
     NSURL *toNSURL() const Q_DECL_NS_RETURNS_AUTORELEASED;
+#  endif
 #endif
 
 #if QT_DEPRECATED_SINCE(5,0)

src/corelib/tools/qstring.h

@@ -65,8 +65,10 @@ namespace std
 #endif
 
 #ifdef Q_OS_MAC
-Q_FORWARD_DECLARE_OBJC_CLASS(NSString);
 Q_FORWARD_DECLARE_CF_TYPE(CFString);
+#  ifdef __OBJC__
+Q_FORWARD_DECLARE_OBJC_CLASS(NSString);
+#  endif
 #endif
 
 QT_BEGIN_NAMESPACE
@@ -681,8 +683,10 @@ public:
 #if defined(Q_OS_MAC) || defined(Q_QDOC)
     static QString fromCFString(CFStringRef string);
     CFStringRef toCFString() const Q_DECL_CF_RETURNS_RETAINED;
+#  if defined(__OBJC__) || defined(Q_QDOC)
     static QString fromNSString(const NSString *string);
     NSString *toNSString() const Q_DECL_NS_RETURNS_AUTORELEASED;
+#  endif
 #endif
     // compatibility
     struct Null { };