scons: New build= option, with support for checked builds.

Where checked build is compiler optimizations plus debugging checks --
ideal for testing CPU bound loads and running test automation loads.
diff --git a/SConstruct b/SConstruct
index 02e99f6..558ebf9 100644
--- a/SConstruct
+++ b/SConstruct
@@ -208,7 +208,7 @@
 
 SConscript(
 	'src/SConscript',
-	variant_dir = env['build'],
+	variant_dir = env['build_dir'],
 	duplicate = 0 # http://www.scons.org/doc/0.97/HTML/scons-user/x2261.html
 )