Add WS_DEPRECATED_X() function attribute

This adds a new attribute that allows declaring Wireshark
functions as deprecated.

Also disabe -Werror with deprecated declarations  Deprecated
declarations can be introduced suddenly with a new version
of an external dependency or a new internal deprecation and
that has its own timeline to fix. We should still be able to
build with -Werror in that case.

CMakeLists.txt

@@ -669,6 +669,13 @@ else() # ! MSVC
 		# seem to flag very interesting issues.
 		#
 		-Wno-format-truncation
+		#
+		# Updating external dependencies can introduce new deprecations.
+		# Also fixing new internal deprecations takes time.
+		# We want to be able to build with -Werror in that case. New
+		# code should not introduce new deprecations in any case.
+		#
+		-Wno-error=deprecated-declarations
 	)
 
 	#

include/ws_attributes.h

@@ -75,6 +75,27 @@ extern "C" {
   #define WS_RETNONNULL
 #endif
 
+/*
+ * WS_DEPRECATED, before a function declaration, means "this function
+ * should not be used anymore and will be removed in a future version".
+ * WS_DEPRECATED_X() optionally takes a message saying what should be done
+ * instead (strongly recommended).
+ *
+ * This is not implemented on purpose with MSVC because that compiler has no
+ * equivalent to -Wno-error=deprecated-declarations, making it impossible
+ * to build with -Werror and deprecated declarations. The Microsoft developer
+ * team seems to not understand the requirement.
+ * https://developercommunity.visualstudio.com/t/cant-treat-deprecated-warning-as-warning-with-wx/786502
+ * https://developercommunity.visualstudio.com/t/impossible-to-treat-warning-as-error-except-specif/473936
+ */
+#if __has_attribute(deprecated)
+  #define WS_DEPRECATED         __attribute__((deprecated))
+  #define WS_DEPRECATED_X(msg)  __attribute__((deprecated(msg)))
+#else
+  #define WS_DEPRECATED
+  #define WS_DEPRECATED_X(msg)
+#endif
+
 /*
  * WS_THREAD_LOCAL means "this variable should go in thread-local
  * storage.

ui/qt/widgets/detachable_tabwidget.cpp

@@ -155,9 +155,7 @@ void DragDropTabBar::mouseMoveEvent(QMouseEvent *event)
         _dragInitiated = true;
 
     if ((event->buttons() & Qt::LeftButton) && _dragInitiated) {
-DIAG_WARN(deprecated-declarations)
         QMouseEvent * finishMouseMove = new QMouseEvent(QEvent::MouseMove, event->pos(), Qt::NoButton, Qt::NoButton, Qt::NoModifier);
-DIAG_ON(deprecated-declarations)
         QTabBar::mouseMoveEvent(finishMouseMove);
 
         QDrag * drag = new QDrag(this);