scons: Debug build by default.

Match what autotools and other build systems do by default.
diff --git a/common.py b/common.py
index a687efc..2d1b258 100644
--- a/common.py
+++ b/common.py
@@ -53,7 +53,7 @@
 		from SCons.Variables.EnumVariable import EnumVariable as EnumOption
 	except ImportError:
 		from SCons.Options.EnumOption import EnumOption
-	opts.Add(BoolOption('debug', 'debug build', 'no'))
+	opts.Add(BoolOption('debug', 'debug build', 'yes'))
 	opts.Add(BoolOption('profile', 'profile build', 'no'))
 	opts.Add(BoolOption('quiet', 'quiet command lines', 'yes'))
 	opts.Add(EnumOption('machine', 'use machine-specific assembly code', default_machine,
diff --git a/scons/generic.py b/scons/generic.py
index 29ddf76..0b3ef5a 100644
--- a/scons/generic.py
+++ b/scons/generic.py
@@ -264,7 +264,7 @@
         pass
 
     # Build type
-    env['debug'] = _bool_map[ARGUMENTS.get('debug', 'no')]
+    env['debug'] = _bool_map[ARGUMENTS.get('debug', 'yes')]
     env['profile'] = _bool_map[ARGUMENTS.get('profile', 'no')]
 
     # Put build output in a separate dir, which depends on the current