[Ada] Take into account GNSA_ROOT env var for prefix

The GNSA_ROOT environment variable can be used to indicate to the
compiler how to find the runtime directory.

gcc/ada/

	* osint.ads, osint.adb (Relocate_Path): If the GNSA_ROOT
	environment variable is set, we use that as the prefix, instead
	of computing the prefix from the executable location.

gcc/ada/osint.adb

@@ -2758,7 +2758,25 @@ package body Osint is
 
    begin
       if Std_Prefix = null then
-         Std_Prefix := Executable_Prefix;
+         Std_Prefix := String_Ptr (Getenv ("GNSA_ROOT"));
+
+         if Std_Prefix.all = "" then
+            Std_Prefix := Executable_Prefix;
+
+         elsif not Is_Directory_Separator (Std_Prefix (Std_Prefix'Last)) then
+
+            --  The remainder of this function assumes that Std_Prefix
+            --  terminates with a dir separator, so we force this here.
+
+            declare
+               Old_Prefix : String_Ptr := Std_Prefix;
+            begin
+               Std_Prefix := new String (1 .. Old_Prefix'Length + 1);
+               Std_Prefix (1 .. Old_Prefix'Length) := Old_Prefix.all;
+               Std_Prefix (Old_Prefix'Length + 1) := Directory_Separator;
+               Free (Old_Prefix);
+            end;
+         end if;
 
          if Std_Prefix.all /= "" then
 

gcc/ada/osint.ads

@@ -237,8 +237,11 @@ package Osint is
    --  replace the Prefix substring with the root installation directory.
    --  By default, try to compute the root installation directory by looking
    --  at the executable name as it was typed on the command line and, if
-   --  needed, use the PATH environment variable. If the above computation
-   --  fails, return Path. This function assumes Prefix'First = Path'First.
+   --  needed, use the PATH environment variable. If the GNSA_ROOT environment
+   --  variable is set, then the content of this variable is used as the root
+   --  installation directory.
+   --  If the above computation fails, return Path. This function assumes
+   --  Prefix'First = Path'First.
 
    function Shared_Lib (Name : String) return String;
    --  Returns the runtime shared library in the form -l<name>-<version> where