feat(3p/glog): Check in derivation for glog

Instead of overriding the upstream one, lets keep it local.

third_party/glog/default.nix

@@ -0,0 +1,18 @@
+{ pkgs, lib, ... }:
+
+let inherit (pkgs) llvmPackages cmake;
+in llvmPackages.libcxxStdenv.mkDerivation {
+  name = "glog";
+  version = "20200527-unstable";
+  src = ./.;
+
+  nativeBuildInputs = [ cmake ];
+  cmakeFlags = [ "-WITH_GFLAGS=OFF" ];
+
+  meta = with lib; {
+    homepage = "https://github.com/google/glog";
+    license = licenses.bsd3;
+    description = "Library for application-level logging";
+    platforms = platforms.unix;
+  };
+}