Fix undeclared identifer when build the editor

Before this change, if you tried to compile Descent 3 with
-D BUILD_EDITOR=ON, then it would fail with this error:

  error C2065: 'Base_directory': undeclared identifier

editor/MainFrm.cpp

@@ -1460,7 +1460,7 @@ void InitCScripts() {
   CreateNewMine();
 
   //	Setup include directories for OSIRIS
-  ddio_MakePath(path, Base_directory, "data", "levels", NULL);
+  ddio_MakePath(path, GetWritableBaseDirectory().string().c_str(), "data", "levels", NULL);
 }
 
 // Copied from winmain.cpp

editor/editor_lighting.cpp

@@ -871,7 +871,7 @@ void DoRadiosityForRooms() {
 
   if (save_after_bsp) {
     char filename[_MAX_PATH];
-    ddio_MakePath(filename, Base_directory, "BSPSave.D3L", NULL);
+    ddio_MakePath(filename, GetWritableBaseDirectory().string().c_str(), "BSPSave.D3L", NULL);
 
     // Save the level to
     SaveLevel(filename);
@@ -1141,7 +1141,7 @@ void DoRadiosityForRooms() {
   SqueezeLightmaps(0, -1);
 
   char filename[_MAX_PATH + 1];
-  ddio_MakePath(filename, Base_directory, "LightSave.D3L", NULL);
+  ddio_MakePath(filename, GetWritableBaseDirectory().string().c_str(), "LightSave.D3L", NULL);
 
   // Save the level to disk
   SaveLevel(filename);

editor/gameeditor.cpp

@@ -620,7 +620,7 @@ void GameToEditor(bool set_viewer_from_player) {
   if (Temp_level_saved) {
     char filename[_MAX_PATH];
 
-    ddio_MakePath(filename, Base_directory, "GameSave.D3L", NULL); // make explicit path
+    ddio_MakePath(filename, GetWritableBaseDirectory().string().c_str(), "GameSave.D3L", NULL); // make explicit path
     LoadLevel(filename);
     Temp_level_saved = 0;
   }
@@ -760,7 +760,7 @@ void EditorToGame() {
   //	set game working directory
   bool set_size = false;
   ddio_GetWorkingDir(Editor_dir, sizeof(Editor_dir));
-  ddio_SetWorkingDir(Base_directory);
+  ddio_SetWorkingDir(GetWritableBaseDirectory().string().c_str());
 
   Osiris_ResetAllTimers();