AC_PROG_CXX: Add checks for C++11, C++98TR1 and C++98

AC_PROG_CXX: Add checks for C++11, C++98TR1 and C++98

These checks are the C++ equivalent of the existing C
standards checks.
* doc/autoconf.texi (C++ Compiler): Document new behavior.
* lib/autoconf/c.m4 (AC_PROG_CXX): Try for C++11,
falling back to C++98.
(_AC_CXX_STD_TRY, _AC_CXX_CXX98_TEST_HEADER, _AC_CXX_CXX98_TEST_BODY)
(_AC_CXX_CXX11_TEST_HEADER, _AC_CXX_CXX11_TEST_BODY)
(_AC_PROG_CXX_CXX98, _AC_PROG_CXX_CXX11):
New macros.