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/nodes/SConscript

@@ -53,7 +53,8 @@ incs = [
     ]
 incs = ' '.join(incs)
 
-defs = env['BF_GL_DEFINITIONS']
+defs = []
+defs += env['BF_GL_DEFINITIONS']
 
 if env['WITH_BF_PYTHON']:
     incs += ' ' + env['BF_PYTHON_INC']