qt: Replace objc_msgSend with native syntax

configure.ac

@@ -585,7 +585,7 @@ case $host in
      fi
 
      AX_CHECK_LINK_FLAG([[-Wl,-headerpad_max_install_names]], [LDFLAGS="$LDFLAGS -Wl,-headerpad_max_install_names"])
-     CPPFLAGS="$CPPFLAGS -DMAC_OSX"
+     CPPFLAGS="$CPPFLAGS -DMAC_OSX -DOBJC_OLD_DISPATCH_PROTOTYPES=0"
      OBJCXXFLAGS="$CXXFLAGS"
      ;;
    *android*)

src/qt/guiutil.cpp

@@ -58,9 +58,10 @@
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wdeprecated-declarations"
 
-#include <objc/objc-runtime.h>
 #include <CoreServices/CoreServices.h>
 #include <QProcess>
+
+void ForceActivation();
 #endif
 
 namespace GUIUtil {
@@ -360,10 +361,7 @@ bool isObscured(QWidget *w)
 void bringToFront(QWidget* w)
 {
 #ifdef Q_OS_MAC
-    // Force application activation on macOS. With Qt 5.4 this is required when
-    // an action in the dock menu is triggered.
-    id app = objc_msgSend((id) objc_getClass("NSApplication"), sel_registerName("sharedApplication"));
-    objc_msgSend(app, sel_registerName("activateIgnoringOtherApps:"), YES);
+    ForceActivation();
 #endif
 
     if (w) {

src/qt/macdockiconhandler.mm

@@ -1,12 +1,11 @@
-// Copyright (c) 2011-2018 The Bitcoin Core developers
+// Copyright (c) 2011-2019 The Bitcoin Core developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include "macdockiconhandler.h"
 
-#undef slots
-#include <objc/objc.h>
-#include <objc/message.h>
+#include <AppKit/AppKit.h>
+#include <objc/runtime.h>
 
 static MacDockIconHandler *s_instance = nullptr;
 
@@ -21,9 +20,7 @@ bool dockClickHandler(id self, SEL _cmd, ...) {
 }
 
 void setupDockClickHandler() {
-    id app = objc_msgSend((id)objc_getClass("NSApplication"), sel_registerName("sharedApplication"));
-    id delegate = objc_msgSend(app, sel_registerName("delegate"));
-    Class delClass = (Class)objc_msgSend(delegate, sel_registerName("class"));
+    Class delClass = (Class)[[[NSApplication sharedApplication] delegate] class];
     SEL shouldHandle = sel_registerName("applicationShouldHandleReopen:hasVisibleWindows:");
     class_replaceMethod(delClass, shouldHandle, (IMP)dockClickHandler, "B@:");
 }
@@ -44,3 +41,13 @@ void MacDockIconHandler::cleanup()
 {
     delete s_instance;
 }
+
+/**
+ * Force application activation on macOS. With Qt 5.5.1 this is required when
+ * an action in the Dock menu is triggered.
+ * TODO: Define a Qt version where it's no-longer necessary.
+ */
+void ForceActivation()
+{
+    [[NSApplication sharedApplication] activateIgnoringOtherApps:YES];
+}