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/scons/llvm.py b/scons/llvm.py
index d88d6e3..39fbb91 100644
--- a/scons/llvm.py
+++ b/scons/llvm.py
@@ -124,7 +124,7 @@
             # Some of the LLVM C headers use the inline keyword without
             # defining it.
             env.Append(CPPDEFINES = [('inline', '__inline')])
-            if env['debug']:
+            if env['build'] in ('debug', 'checked'):
                 # LLVM libraries are static, build with /MT, and they
                 # automatically link agains LIBCMT. When we're doing a
                 # debug build we'll be linking against LIBCMTD, so disable