SCons: Fix for really nasty bug with polluting configuration environment

The issue was caused by the following construction:

  def = env['SOMETHING']
  defs.append('SOMETHING_MORE')

Since first assignment was actually referencing environment option it was totally
polluted hawing weird and wonderful side effects on all other areas of Blender.

source/blender/editors/space_node/SConscript

@@ -1,3 +1,4 @@
+
 #!/usr/bin/env python
 #
 # ***** BEGIN GPL LICENSE BLOCK *****
@@ -48,7 +49,8 @@ incs = [
     ]
 incs = ' '.join(incs)
 
-defs = env['BF_GL_DEFINITIONS']
+defs = []
+defs += env['BF_GL_DEFINITIONS']
 
 cf = []
 if env['OURPLATFORM'] in ('win32-vc', 'win64-vc'):