new (old) -isystem-cxx flag to set a C++ system include directory.

* c-opts.c: Document -isystem-cxx.
* cppinit.c (new_pending_directive): Add -isystem-cxx entry.
* gcc.h. (DEFAULT_WORD_SWITCH_TAKES_ARG): add -isystem.


XXX: possibly can use $CPLUS_INCLUDE_DIR.

gnu/dist/gcc/gcc/c-opts.c

@@ -1731,6 +1731,10 @@ Switches:\n\
   -iwithprefix <dir>        Add <dir> to the end of the system include path\n\
   -iwithprefixbefore <dir>  Add <dir> to the end of the main include path\n\
   -isystem <dir>            Add <dir> to the start of the system include path\n\
+"), stdout);
+  fputs (_("\
+  -isystem-cxx <dir>        Add <dir> to the start of the system C++ include\n\
+                             path\n\
 "), stdout);
   fputs (_("\
   -idirafter <dir>          Add <dir> to the end of the system include path\n\

gnu/dist/gcc/gcc/cppinit.c

@@ -1179,6 +1179,7 @@ new_pending_directive (pend, text, handler)
   DEF_OPT("include",                  no_fil, OPT_include)                    \
   DEF_OPT("iprefix",                  no_pth, OPT_iprefix)                    \
   DEF_OPT("isystem",                  no_dir, OPT_isystem)                    \
+  DEF_OPT("isystem-cxx",              no_dir, OPT_isystemcxx)                 \
   DEF_OPT("iwithprefix",              no_dir, OPT_iwithprefix)                \
   DEF_OPT("iwithprefixbefore",        no_dir, OPT_iwithprefixbefore)
 
@@ -1362,6 +1363,11 @@ cpp_handle_option (pfile, argc, argv)
 	     include directory.  */
 	  append_include_chain (pfile, xstrdup (arg), SYSTEM, 0);
 	  break;
+	case OPT_isystemcxx:
+	  /* Add directory to beginning of system C++ include path, as a system
+	     include directory.  */
+	  append_include_chain (pfile, xstrdup (arg), SYSTEM, 1);
+	  break;
 	case OPT_include:
 	case OPT_imacros:
 	  {

gnu/dist/gcc/gcc/gcc.h

@@ -48,8 +48,8 @@ struct spec_function
   || !strcmp (STR, "imacros") || !strcmp (STR, "aux-info") \
   || !strcmp (STR, "idirafter") || !strcmp (STR, "iprefix") \
   || !strcmp (STR, "iwithprefix") || !strcmp (STR, "iwithprefixbefore") \
-  || !strcmp (STR, "isystem") || !strcmp (STR, "-param") \
-  || !strcmp (STR, "specs") \
+  || !strcmp (STR, "isystem") || !strcmp (STR, "isystem-cxx") \
+  || !strcmp (STR, "-param") || !strcmp (STR, "specs") \
   || !strcmp (STR, "MF") || !strcmp (STR, "MT") || !strcmp (STR, "MQ"))