QLibrary: fake RLTD_NODELETE by not calling dlclose()

On systems without the RTLD_NODELETE flag, simply don't call dlclose()
and leak the handle. Amends ef5ab6e006.

Pick-to: 6.7
Change-Id: I01ec3c774d9943adb903fffd17b76673562daa8a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>

src/corelib/plugin/qlibrary_unix.cpp

@@ -257,8 +257,12 @@ bool QLibraryPrivate::load_sys()
 
 bool QLibraryPrivate::unload_sys()
 {
-#if !defined(Q_OS_VXWORKS)            // Unloading on VxWorks causes crashes in QtDeclarative autotests
-    if (dlclose(pHnd.loadAcquire())) {
+    bool doTryUnload = true;
+#ifndef RTLD_NODELETE
+    if (loadHints() & QLibrary::PreventUnloadHint)
+        doTryUnload = false;
+#endif
+    if (doTryUnload && dlclose(pHnd.loadAcquire())) {
         const char *error = dlerror();
 #if defined (Q_OS_QNX)
         // Workaround until fixed in QNX; fixes crash in
@@ -271,7 +275,6 @@ bool QLibraryPrivate::unload_sys()
         return false;
     }
     errorString.clear();
-#endif
     return true;
 }