Let the ARI know gdb_Py_DECREF is OK.

The ARI complains with:

> gdb/python/python-internal.h:177: code: editCase function: Function name starts lower case but has uppercased letters.
gdb/python/python-internal.h:177:gdb_Py_DECREF (void *op)

gdb_Py_DECREF is just wrapping a python macro that happens to be mixed case.

gdb/
2013-05-22  Pedro Alves  <palves@redhat.com>

	* python/python-internal.h (gdb_Py_DECREF): Tag with
	"ARI: editCase function".
readline/

	* configure.in: Invoke AC_CANONICAL_BUILD.
	Change $host_os to $build_os.
	* configure: Regenerated.