Prerequisites
Python
If you don't already have it (python external libraries under GLAST_EXT don't count) install python from the official site . Version should be at least 2.5; 2.6 or 2.7 preferred. Do not install anything in the 3.x series.
SCons
You'll need 1.3.0 or newer; 2.1.0 is preferred. You can get it from the official SCons site .