Suppress some Mac-specific linker warnings.

Contributes to RobotLocomotion#5183.

tools/CROSSTOOL

@@ -356,6 +356,20 @@ toolchain {
   tool_path { name: "cpp" path: "/usr/bin/cpp" }
   tool_path { name: "dwp" path: "/usr/bin/dwp" }
   tool_path { name: "gcc" path: "third_party/bazel/tools/cpp/osx_cc_wrapper.sh" }
+
+  ### Additional flags for Drake.
+  ### These must go above the Bazel-derived flags, because some of those flags
+  ### are designed to be concatenated with a predicate, for instance the
+  ### ar_flag "-o".
+  # This warning is common in Drake, and we don't really care about compact
+  # exception unwinding, since catching exceptions is forbidden, so we suppress
+  # it. See https://github.com/RobotLocomotion/drake/issues/5183
+  linker_flag: "-Wl,-no_compact_unwind"
+  # The no-symbols warning is common and harmless for header-only libraries,
+  # so we suppress it. See https://github.com/RobotLocomotion/drake/issues/5183
+  ar_flag: "-no_warning_for_no_symbols"
+  ### End of additional flags for Drake.
+
   cxx_flag: "-std=c++1y"
   ar_flag: "-static"
   ar_flag: "-s"