Adds -lm linker flag for FreeBSD/OpenBSD and uses github.com/bazelbuil… (#1032)

* Adds -lm linker flag for (Free|Open)BSD and uses github.com/bazelbuild/platforms for platform detection.

* Prefer selects.with_or to select the linkopts.

* @platforms appears to be implicitly available. @bazel_skylib would require updating every dependent repository.

* Re-enable platforms package.

BUILD.bazel

@@ -1,14 +1,35 @@
+load("@rules_cc//cc:defs.bzl", "cc_library")
+
 licenses(["notice"])
 
 config_setting(
-    name = "windows",
-    values = {
-        "cpu": "x64_windows",
-    },
+    name = "freebsd",
+    constraint_values = [
+        "@platforms//os:freebsd",
+    ],
     visibility = [":__subpackages__"],
 )
 
-load("@rules_cc//cc:defs.bzl", "cc_library")
+config_setting(
+    name = "openbsd",
+    constraint_values = [
+        "@platforms//os:openbsd",
+    ],
+    visibility = [":__subpackages__"],
+)
+
+config_setting(
+    name = "windows",
+    constraint_values = [
+        "@platforms//os:windows",
+    ],
+    visibility = [":__subpackages__"],
+)
+
+BSD_LINKOPTS = [
+    "-pthread",
+    "-lm",
+]
 
 cc_library(
     name = "benchmark",
@@ -22,6 +43,8 @@ cc_library(
     hdrs = ["include/benchmark/benchmark.h"],
     linkopts = select({
         ":windows": ["-DEFAULTLIB:shlwapi.lib"],
+        ":freebsd": BSD_LINKOPTS,
+        ":openbsd": BSD_LINKOPTS,
         "//conditions:default": ["-pthread"],
     }),
     strip_include_prefix = "include",

WORKSPACE

@@ -35,3 +35,8 @@ new_local_repository(
     path = "/usr/include/python3.6",  # May be overwritten by setup.py.
 )
 
+http_archive(
+    name = "platforms",
+    strip_prefix = "platforms-master",
+    urls = ["https://github.com/bazelbuild/platforms/archive/master.zip"],
+)