Applying this fix:

engine/SCons/Platform/win32.py

Index: engine/SCons/Platform/win32.py
===================================================================
--- engine/SCons/Platform/win32.py	(revision 4802)
+++ engine/SCons/Platform/win32.py	(working copy)
@@ -41,6 +41,33 @@
 from SCons.Platform import TempFileMunge
 import SCons.Util
 
+#NACL_CHANGE(bradnelson) - FROM THIS:
+#NACL_CHANGE(bradnelson) - TO THIS:
+import threading
+
+# Monkey patch os.spawnve on Windows to become thread safe
+from os import spawnve as old_spawnve
+
+spawn_lock = threading.Lock()
+
+def new_spawnve(mode, file, args, env):
+    spawn_lock.acquire()
+    try:
+        if mode == os.P_WAIT:
+            ret = old_spawnve(os.P_NOWAIT, file, args, env)
+        else:
+            ret = old_spawnve(mode, file, args, env)
+    finally:
+        spawn_lock.release()
+    if mode == os.P_WAIT:
+        pid, status = os.waitpid(ret, 0)
+        ret = status >> 8
+    return ret
+
+os.spawnve = new_spawnve
+
+#NACL_CHANGE(bradnelson) - END CHANGES.
+
 try:
     import msvcrt
     import win32api