Clean up post-C++-17 upgrade

-There was a TODO to remove a const_cast
-Remove a bunch of -pthread uses, because they were apparently unneeded.

Change-Id: I4cd28ccfb487eb0c7acb3b629e090c5e8a1b2acc
diff --git a/third_party/abseil/absl/debugging/BUILD.bazel b/third_party/abseil/absl/debugging/BUILD.bazel
index 01e2ea2..3df2a8f 100644
--- a/third_party/abseil/absl/debugging/BUILD.bazel
+++ b/third_party/abseil/absl/debugging/BUILD.bazel
@@ -122,10 +122,7 @@
     name = "failure_signal_handler_test",
     srcs = ["failure_signal_handler_test.cc"],
     copts = ABSL_TEST_COPTS,
-    linkopts = select({
-        "//absl:windows": [],
-        "//conditions:default": ["-pthread"],
-    }) + ABSL_DEFAULT_LINKOPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":failure_signal_handler",
         ":stacktrace",