gdb: better static python detection in configure machinery

In python 3, itertools is a builtin module, so whether or not the
python you link against is a shared or a static one, importing it
works.

Change the import test to use ctypes which is a dynamic module in both
python 2 and 3.

gdb/ChangeLog:

	PR python/26832
	* configure: Regenerate.
	* configure.ac: Check for python modules ctypes instead of
	itertools.