updates re C++ standards

git-svn-id: https://svn.r-project.org/R/trunk@89078 00db46b3-68df-0310-9c12-caf00c1e9a41

Author: ripley

doc/manual/R-admin.texi

@@ -1041,9 +1041,9 @@ C++17 tests, so do this at your own risk as some packages may not compile.
 The @option{-std} flag is supported by the GCC, @command{clang++} and
 Intel compilers.  Currently accepted values are (plus some synonyms)
 @example
-g++:     c++17 gnu++17 c++2a gnu++2a (from 8)
-         c++20 gnu++20 (from 10) c++23 gnu++23 c++2b gnu++2b (from 11)
-         c++2c gnu++2c
+g++:     c++17 gnu++17 (from 8) c++20 gnu++20 (from 10)
+         c++23 gnu++23 c++2b gnu++2b (from 11)
+         c++2c gnu++2c c++26 gnu++26 (from 14)
 Intel:   c++17 gnu++17 c++20 gnu++20 (from 2021.1)
          c++2b gnu++2b (from 2022.2) c++23 gnu++23 (at least from 2024.0)
 @end example
@@ -1052,8 +1052,7 @@ Intel:   c++17 gnu++17 c++20 gnu++20 (from 2021.1)
 @uref{https://clang.llvm.org/cxx_status.html}, and follow @code{g++}:
 @code{-std=c++20} is supported from Clang 10, @code{-std=c++2b} from
 Clang 13 and @code{-std=c++23} and @code{-std=c++2c} from Clang 17.
-Apple Clang supports @code{-std=c++2b} from 13.1.6 and @code{-std=c++23}
-and @code{-std=c++2c} from 16.0.0, @code{-std=c++26} from 17.0.0.)
+Apple Clang supports @code{-std=c++23} and @code{-std=c++26} from 16.0.0.
 
 @c c++1y does not pass the configure test in gcc 4.9.3
 @c , with @option{-std=c++1y} (introduced@footnote{See