--- a/PsychSourceGL/Source/linuxmakeitoctave3.m
+++ b/PsychSourceGL/Source/linuxmakeitoctave3.m
@@ -13,6 +13,9 @@ if nargin < 1
     mode = 0;
 end;
 
+% In-source-tree building
+PsychtoolboxRoot = '../../Psychtoolbox/'
+
 fprintf('Building plugin type %i ...\n\n', mode);
 
 if mode==0
