Disable signed-compare warnings in TensorFlow default build.

We have a lot of such warnings and don't make an attempt to clean them up
internally.

PiperOrigin-RevId: 216631010
diff --git a/configure.py b/configure.py
index 89dc79b..07c6e41 100644
--- a/configure.py
+++ b/configure.py
@@ -497,7 +497,7 @@
   elif is_windows():
     default_cc_opt_flags = '/arch:AVX'
   else:
-    default_cc_opt_flags = '-march=native'
+    default_cc_opt_flags = '-march=native -Wno-sign-compare'
   question = ('Please specify optimization flags to use during compilation when'
               ' bazel option "--config=opt" is specified [Default is %s]: '
              ) % default_cc_opt_flags