Support $SYSROOT for = in -I etc.

	* incpath.c (add_sysroot_to_chain): Allow for $SYSROOT prefix.
	* doc/cppdiropts.texi (-I @var{dir}): Document it.

From-SVN: r249560

gcc/ChangeLog

+2017-06-22  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>
+
+	* incpath.c (add_sysroot_to_chain): Allow for $SYSROOT prefix.
+	* doc/cppdiropts.texi (-I @var{dir}): Document it.
+
 2016-06-22  Richard Biener  <rguenther@suse.de>
 
 	* tree-vect-loop.c (vect_model_reduction_cost): Handle

gcc/doc/cppdiropts.texi

@@ -22,8 +22,9 @@ for header files during preprocessing.
 @ifset cppmanual
 @xref{Search Path}.
 @end ifset
-If @var{dir} begins with @samp{=}, then the @samp{=} is replaced
-by the sysroot prefix; see @option{--sysroot} and @option{-isysroot}.
+If @var{dir} begins with @samp{=} or @code{$SYSROOT}, then the @samp{=}
+or @code{$SYSROOT} is replaced by the sysroot prefix; see
+@option{--sysroot} and @option{-isysroot}.
 
 Directories specified with @option{-iquote} apply only to the quote 
 form of the directive, @code{@w{#include "@var{file}"}}.

gcc/incpath.c

@@ -314,7 +314,7 @@ remove_duplicates (cpp_reader *pfile, struct cpp_dir *head,
 }
 
 /* Add SYSROOT to any user-supplied paths in CHAIN starting with
-   "=".  */
+   "=" or "$SYSROOT".  */
 
 static void
 add_sysroot_to_chain (const char *sysroot, int chain)
@@ -322,8 +322,15 @@ add_sysroot_to_chain (const char *sysroot, int chain)
   struct cpp_dir *p;
 
   for (p = heads[chain]; p != NULL; p = p->next)
-    if (p->name[0] == '=' && p->user_supplied_p)
-      p->name = concat (sysroot, p->name + 1, NULL);
+    {
+      if (p->user_supplied_p)
+	{
+	  if (p->name[0] == '=')
+	    p->name = concat (sysroot, p->name + 1, NULL);
+	  if (strncmp (p->name, "$SYSROOT", strlen ("$SYSROOT")) == 0)
+	    p->name = concat (sysroot, p->name + strlen ("$SYSROOT"), NULL);
+	}
+    }
 }
 
 /* Merge the four include chains together in the order quote, bracket,