gobject.py: Don't install frame filters when GDB does not support them

Stock GDB (both versions 7.0 and 7.1) does not come with the new
backtrace code and python API. To prevent an ugly python backtrace when
auto-loading gobject.py, let's catch the exception and not register the
FrameWrapper and the FrameFilter.

https://bugzilla.gnome.org/show_bug.cgi?id=613732

gobject/gobject.py

+import os.path
 import gdb
 import glib
-import gdb.backtrace
-import gdb.command.backtrace
+try:
+    import gdb.backtrace
+    import gdb.command.backtrace
+except ImportError:
+    print(os.path.basename(__file__) + ": gdb was not built with "
+          "custom backtrace support, disabling.")
+    HAVE_GDB_BACKTRACE = 0
+else:
+    HAVE_GDB_BACKTRACE = 1
 
 # This is not quite right, as local vars may override symname
 def read_global_var (symname):
@@ -107,13 +115,14 @@ class GFrameWrapper:
         return getattr (self.frame, name)
 
 # Monkey patch FrameWrapper to avoid IA__ in symbol names
-old__init__ = gdb.command.backtrace.FrameWrapper.__init__
-def monkey_patched_init(self, frame):
-    name = frame.name()
-    if name and name.startswith("IA__"):
-        frame = GFrameWrapper(frame)
-    old__init__(self,frame)
-gdb.command.backtrace.FrameWrapper.__init__ = monkey_patched_init
+if HAVE_GDB_BACKTRACE:
+    old__init__ = gdb.command.backtrace.FrameWrapper.__init__
+    def monkey_patched_init(self, frame):
+        name = frame.name()
+        if name and name.startswith("IA__"):
+            frame = GFrameWrapper(frame)
+        old__init__(self,frame)
+    gdb.command.backtrace.FrameWrapper.__init__ = monkey_patched_init
 
 class DummyFrame:
     def __init__ (self, frame):
@@ -301,5 +310,6 @@ def register (obj):
     if obj == None:
         obj = gdb
 
-    gdb.backtrace.push_frame_filter (GFrameFilter)
+    if HAVE_GDB_BACKTRACE:
+        gdb.backtrace.push_frame_filter (GFrameFilter)
     obj.pretty_printers.append(pretty_printer_lookup)