Add notes on -stdlib=libc++ for MacOS

GNUmakefile

@@ -604,10 +604,14 @@ ifeq ($(IS_LINUX),1)
   endif # OpenMP
 endif # IS_LINUX
 
+# libc++ is LLVM's standard C++ library. If we add libc++
+# here then all user programs must use it too. The open
+# question is, which choice is easier on users?
 ifneq ($(IS_DARWIN),0)
+  CXX ?= c++
+  # CXXFLAGS += -stdlib=libc++
   AR = libtool
   ARFLAGS = -static -o
-  CXX ?= c++
 endif
 
 # Add -errtags=yes to get the name for a warning suppression