Compatibility fixes for scons using Python 3

Use print style compatible with Python 2/3.
Set text mode in popen so that textual regexps work.

Change-Id: I079587afd0cf6c896ff5adb5dda3f2bc1fdeff4a
diff --git a/SConstruct b/SConstruct
index 8e1b1a7..42c1c25 100644
--- a/SConstruct
+++ b/SConstruct
@@ -459,11 +459,11 @@
 # Abort the build if any command line option is unknown or invalid.
 unknown_build_options = vars.UnknownVariables()
 if unknown_build_options:
-  print 'Unknown build options:',  unknown_build_options.keys()
+  print('Unknown build options: ' + str(unknown_build_options.keys()))
   Exit(1)
 
 if env['negative_testing'] == 'on' and env['mode'] != 'debug':
-  print 'negative_testing only works in debug mode'
+  print('negative_testing only works in debug mode')
   Exit(1)
 
 ConfigureEnvironment(env)