ansi-c++-opt: Provide option --enable-c++/--disable-c++ when possible.
* m4/ansi-c++.m4 (gl_CXX_CHOICE): In Autoconf 2.66 or newer, provide
option --enable/disable-c++ instead of --enable/disable-cxx.
+ * NEWS: Mention the change.
2010-07-31 Bruno Haible <bruno@clisp.org>