Fix linker errors on (Free|Open)BSD. (#6860)

* Fix linker errors on (Free|Open)BSD.

* Adds a TODO for the FreeBSD linker flags: -lm and moves platform config to main BUILD.bazel

BUILD.bazel

@@ -10,6 +10,20 @@ exports_files([
     "LICENSE",
 ])
 
+config_setting(
+    name = "platform_freebsd",
+    constraint_values = [
+        "@platforms//os:freebsd",
+    ],
+)
+
+config_setting(
+    name = "platform_openbsd",
+    constraint_values = [
+        "@platforms//os:openbsd",
+    ],
+)
+
 # Public flatc library to compile flatbuffer files at runtime.
 cc_library(
     name = "flatbuffers",

src/BUILD.bazel

@@ -17,6 +17,15 @@ cc_library(
         "util.cpp",
     ],
     hdrs = ["//:public_headers"],
+    linkopts = select({
+        # TODO: Bazel uses `clang` instead of `clang++` to link
+        # C++ code on BSD. Temporarily adding these linker flags while
+        # we wait for Bazel to resolve
+        # https://github.com/bazelbuild/bazel/issues/12023.
+        "//:platform_freebsd": ["-lm"],
+        "//:platform_openbsd": ["-lm"],
+        "//conditions:default": [],
+    }),
     strip_include_prefix = "/include",
     visibility = ["//:__pkg__"],
 )