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",
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",
diff --git a/third_party/abseil/absl/random/internal/BUILD.bazel b/third_party/abseil/absl/random/internal/BUILD.bazel
index ec4fa53..d394045 100644
--- a/third_party/abseil/absl/random/internal/BUILD.bazel
+++ b/third_party/abseil/absl/random/internal/BUILD.bazel
@@ -109,10 +109,7 @@
         "pool_urbg.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
-    linkopts = select({
-        "//absl:windows": [],
-        "//conditions:default": ["-pthread"],
-    }) + ABSL_DEFAULT_LINKOPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":randen",
         ":seed_material",
diff --git a/third_party/abseil/absl/synchronization/BUILD.bazel b/third_party/abseil/absl/synchronization/BUILD.bazel
index 89d5538..6d640ae 100644
--- a/third_party/abseil/absl/synchronization/BUILD.bazel
+++ b/third_party/abseil/absl/synchronization/BUILD.bazel
@@ -87,10 +87,7 @@
         "notification.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
-    linkopts = select({
-        "//absl:windows": [],
-        "//conditions:default": ["-pthread"],
-    }) + ABSL_DEFAULT_LINKOPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":graphcycles_internal",
         ":kernel_timeout_internal",
diff --git a/third_party/allwpilib_2019/BUILD b/third_party/allwpilib_2019/BUILD
index cfae703..91a3d87 100644
--- a/third_party/allwpilib_2019/BUILD
+++ b/third_party/allwpilib_2019/BUILD
@@ -80,7 +80,6 @@
     ],
     defines = ["WPILIB2019=1"],
     includes = _hal_header_dirs,
-    linkopts = ["-lpthread"],
     restricted_to = ["//tools:roborio"],
     visibility = ["//third_party:__pkg__"],
     deps = [
diff --git a/third_party/google-benchmark/BUILD.bazel b/third_party/google-benchmark/BUILD.bazel
index 7ba1887..479d59e 100644
--- a/third_party/google-benchmark/BUILD.bazel
+++ b/third_party/google-benchmark/BUILD.bazel
@@ -24,7 +24,7 @@
     ],
     linkopts = select({
         ":windows": ["-DEFAULTLIB:shlwapi.lib"],
-        "//conditions:default": ["-pthread"],
+        "//conditions:default": [],
     }),
     strip_include_prefix = "include",
     visibility = ["//visibility:public"],
diff --git a/third_party/googletest/googletest/test/BUILD.bazel b/third_party/googletest/googletest/test/BUILD.bazel
index 0a66ce9..7d03cac 100644
--- a/third_party/googletest/googletest/test/BUILD.bazel
+++ b/third_party/googletest/googletest/test/BUILD.bazel
@@ -107,13 +107,6 @@
         "googletest/include/internal",
         "googletest/test",
     ],
-    linkopts = select({
-        "//third_party/googletest:windows": [],
-        "//third_party/googletest:windows_msvc": [],
-        "//conditions:default": [
-            "-pthread",
-        ],
-    }),
     deps = ["//third_party/googletest:gtest_main"],
 )