fix generator.py call in scons 3

scons 3 still untested

SConstruct

@@ -1,12 +1,12 @@
 import sys
 #run generator.py
-if sys.version_info[0] >= 3:
-	def execfile(path):
-		with open(path, 'r') as f:
-			exec(f.read())
-
 if not GetOption('clean'):
-	execfile("generator.py")
+	if sys.version_info[0] < 3:
+		execfile("generator.py")
+	else:
+		with open("generator.py") as f:
+			code = compile(f.read(), "generator.py", 'exec')
+			exec(code)
 
 AddOption('--builddir',dest="builddir",default="build",help="Directory to build to.")
 SConscript('SConscript', variant_dir=GetOption('builddir'), duplicate=0)