more GNU make gotchas

git-svn-id: https://svn.r-project.org/R/trunk@85833 00db46b3-68df-0310-9c12-caf00c1e9a41

doc/manual/R-admin.texi

@@ -1093,15 +1093,13 @@ Clang 13 and @code{-std=c++23} from Clang 17.  Apple Clang supports
 `Standards' for @command{g++} starting with @samp{gnu} enable `GNU
 extensions': what those are is hard to track down.
 
-For the use of C++11 and later in @R{} packages see the `Writing R
+For the use of C++ in @R{} packages see the `Writing R
 Extensions' manual.  Prior to @R{} 3.6.0 the default C++ standard was
-that of the compiler used: currently it is C++17 (if available): this
-can be overridden by setting @samp{CXXSTD} when @R{} is configured.
+that of the compiler used: currently it is C++17 (if available).
 
 @uref{https://en.cppreference.com/w/cpp/compiler_support} indicates
 which versions of common compilers support (parts of) which C++
-standards.  GCC 5 was the minimum version with sufficient C++14
-support. GCC introduced C++17 support gradually, but version 7 should
+standards.  GCC introduced C++17 support gradually, but version 7 should
 suffice.
 
 @node C standards, Link-Time Optimization, C++ Support, Other Options

doc/manual/R-exts.texi

@@ -4917,7 +4917,13 @@ platforms but do not have the intended results.
 
 Note that the @option{-C} flag for @command{make} is not included in the
 POSIX specification and is not implemented by some of the
-@command{make}s which have been used with @R{}.
+@command{make}s which have been used with @R{}.  However, it is more
+commonly implemented (e.g.@: by FreeBSD @command{make}) than the GNU-specific
+@option{--directory=}.
+
+You should not rely on built-in/default @command{make} rules, even when
+specified by POSIX, as some @command{make}s do not have the POSIX ones
+and others have altered them.
 
 The use of @code{$@{shell ...@}} can be avoided by using backticks, e.g.@: