Make cereal thread safe by default (#178)

# Changes

If you read the [Cereal library
documentation](https://uscilab.github.io/cereal/thread_safety.html), you
will be shocked that the default behavior for the library is to not have
it run in a multithreaded manner. It apparently has a global variable
that it uses to track information and it works nicely in a single
threaded application, however in a mutlithreaded application is can be
problematic.

The changes here changes the default behavior so that this global
variable is thread safe and doesn't cause any undefined behavior.

Author: RReichert

third_party/cereal.BUILD

@@ -15,5 +15,7 @@ package(
 cc_library(
     name = "cereal",
     hdrs = glob(["include/**"]),
+    defines = ["CEREAL_THREAD_SAFE=1"],
     includes = ["include"],
+    linkopts = ["-lpthread"],
 )