Implement and document Objective C++ support.

Implement and document Objective C++ support.

* lib/autoconf/c.m4 (AC_LANG(Objective C++), AC_LANG_OBJCXX)
(AC_LANG_PREPROC(Objective C++), AC_PROG_OBJCXXCPP)
(AC_LANG_COMPILER(Objective C++), AC_PROG_OBJCXX)
(_AC_PROG_OBJCXX_G): New macros.
(_AC_ARG_VAR_CPPFLAGS, _AC_ARG_VAR_LDFLAGS)
(_AC_ARG_VAR_LIBS): Adjusted.
* doc/autoconf.texi (Objective C++ Compiler): New node.
(Preset Output Variables): Document OBJCXXFLAGS.
(Language Choice): Document `Objective C++' language.
* NEWS: Updated.

* tests/local.at (AT_CHECK_ENV): Ignore AC_SUBSTed Objective C++
related variables.

Signed-off-by: Peter Breitenlohner <peb@mppmu.mpg.de>
Signed-off-by: Eric Blake <ebb9@byu.net>