Avoid non-C++-enabled babeltrace versions

Avoid non-C++-enabled babeltrace versions

In some babeltrace versions before 1.2.0, the header file iterator.h
declares the enum values `BT_SEEK_*' within the struct declaration of
bt_iter_pos.  The enum values are supposed to be globally-scoped, which
works for C, but not for C++.  Later babeltrace versions declare the
enum outside the struct:

  https://lists.lttng.org/pipermail/lttng-dev/2013-September/021411.html

Now that GDB is compiled with C++, the GDB build fails on a system with
an affected babeltrace version: the compiler complains about a missing
declaration of BT_SEEK_BEGIN in ctf.c.

This patch enhances the configure check to recognize such babeltrace
versions as unusable for GDB.

gdb/ChangeLog:

	* configure.ac: Enhance configure check for babeltrace to reject
	non-C++-enabled versions.
	* configure: Regenerate.