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/base/BUILD.bazel b/third_party/abseil/absl/base/BUILD.bazel
index 762e954..b7754de 100644
--- a/third_party/abseil/absl/base/BUILD.bazel
+++ b/third_party/abseil/absl/base/BUILD.bazel
@@ -132,10 +132,7 @@
         "internal/low_level_alloc.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
-    linkopts = select({
-        "//absl:windows": [],
-        "//conditions:default": ["-pthread"],
-    }) + ABSL_DEFAULT_LINKOPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = [
         "//absl:__subpackages__",
     ],
@@ -192,7 +189,7 @@
     copts = ABSL_DEFAULT_COPTS,
     linkopts = select({
         "//absl:windows": [],
-        "//conditions:default": ["-pthread"],
+        "//conditions:default": [],
     }) + ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":atomic_hook",