Remove -Bsymolink link option on macOS. (#10)

This removes the hardcoded link option -Bsymbolic which is specific to
GNU's linker when running on macOS.

build_defs.bzl

diff --git a/build_defs.bzl b/build_defs.bzl
index 651b800..f857776 100644
--- a/build_defs.bzl
+++ b/build_defs.bzl
@@ -39,9 +39,10 @@
         name = name + ".so",
         copts = copts + PYBIND_COPTS + ["-fvisibility=hidden"],
         features = features + PYBIND_FEATURES,
-        linkopts = [
-            "-Wl,-Bsymbolic",
-        ],
+        linkopts = select({
+            "@pybind11//:darwin": ["-Wl"],
+            "//conditions:default": ["-Wl,-Bsymbolic"],
+        }),
         linkshared = 1,
         tags = tags + ["local", "manual"],
         deps = deps + PYBIND_DEPS,

pybind11.BUILD

diff --git a/pybind11.BUILD b/pybind11.BUILD
index 1367827..ce0fc70 100644
--- a/pybind11.BUILD
+++ b/pybind11.BUILD
@@ -34,3 +34,9 @@
     includes = ["include"],
     deps = ["@local_config_python//:python_headers"],
 )
+
+config_setting(
+    name = "darwin",
+    values = {"cpu": "darwin"},
+    visibility = ["//visibility:public"],
+)