[C++] Default to -Werror in C++ mode too

Both x86_64 GNU/Linux and x86_64 mingw-w64 build cleanly with
--enable-targets=all.  This enables -Werror by default in C++ mode
too, in order to let the buildbot catch C++ build regressions for us.

gdb/ChangeLog:
2015-11-19  Pedro Alves  <palves@redhat.com>

	* configure.ac (ERROR_ON_WARNING): Don't check whether in C++
	mode.
	* configure: Regenerate.

gdb/gdbserver/ChangeLog:
2015-11-19  Pedro Alves  <palves@redhat.com>

	* configure.ac (ERROR_ON_WARNING): Don't check whether in C++
	mode.
	* configure: Regenerate.