Execute Build Targets in current directory

When a directory is not explictly specified by the user
use GS's current directory to execute build targets, instead
of the project's root directory.

This is needed since users might change the current directory
themselves, using the GPS.cd Python function for instance.

For eng/ide/gnatstudio#129

Author: AnthonyLeonardoGracio

builder/src/build_command_manager-end_of_build.adb

@@ -235,8 +235,7 @@ package body Build_Command_Manager.End_Of_Build is
          if Directory /= No_File then
             Build.Full.Dir := Directory;
          else
-            Build.Full.Dir :=
-              Builder.Kernel.Registry.Tree.Root_Project.Project_Path.Dir;
+            Build.Full.Dir := GNATCOLL.VFS.Get_Current_Dir;
          end if;
       end if;
    end Local_Expand_Command_Line;