Add DeepBind option to QLibrary

Adds an option to request the RTLD_DEEPBIND flag to dlopen. On Linux
this can be used to force a library to resolve global symbols locally
instead of using the similarly named symbols already loaded.

This makes it possible to load and use plugins linked against Qt 4
without crashing.

[ChangeLog][QtCore][QLibrary] Added DeepBindHint which maps to RTLD_DEEPBIND
on Linux making it possible to load libraries with external symbols that
clash with already loaded ones, such as plugins linked to Qt4.

Change-Id: I4edb4af68e4a47e932a87d108360dba8d91dc34a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/plugin/qlibrary.cpp

@@ -168,6 +168,11 @@ QT_BEGIN_NAMESPACE
     Prevents the library from being unloaded from the address space if close()
     is called. The library's static variables are not reinitialized if open()
     is called at a later time.
+    \value DeepBindHint
+    Instructs the linker to prefer definitions in the loaded library
+    over exported definitions in the loading application when resolving
+    external symbols in the loaded library. This option is only supported
+    on Linux.
 
     \sa loadHints
 */

src/corelib/plugin/qlibrary.h

@@ -53,7 +53,8 @@ public:
         ResolveAllSymbolsHint = 0x01,
         ExportExternalSymbolsHint = 0x02,
         LoadArchiveMemberHint = 0x04,
-        PreventUnloadHint = 0x08
+        PreventUnloadHint = 0x08,
+        DeepBindHint = 0x10
     };
     Q_DECLARE_FLAGS(LoadHints, LoadHint)
 

src/corelib/plugin/qlibrary_unix.cpp

@@ -171,6 +171,10 @@ bool QLibraryPrivate::load_sys()
         dlFlags |= RTLD_LOCAL;
     }
 #endif
+#if defined(RTLD_DEEPBIND)
+    if (loadHints & QLibrary::DeepBindHint)
+        dlFlags |= RTLD_DEEPBIND;
+#endif
 
     // Provide access to RTLD_NODELETE flag on Unix
     // From GNU documentation on RTLD_NODELETE: