fix(3p/nix): Link to glog built by Nix

Same as cl/921, it seems that the trick to making this work is indeed
overriding the C++ standard used in the dependency.

Change-Id: I3c5984d71014d774c161ecc283844f504fd44719
Reviewed-on: https://cl.tvl.fyi/c/depot/+/922
Tested-by: BuildkiteCI
Reviewed-by: lukegb <lukegb@tvl.fyi>

third_party/glog/default.nix

@@ -7,7 +7,10 @@ in llvmPackages.libcxxStdenv.mkDerivation {
   src = ./.;
 
   nativeBuildInputs = [ cmake ];
-  cmakeFlags = [ "-WITH_GFLAGS=OFF" ];
+  cmakeFlags = [
+    "-DCMAKE_CXX_STANDARD=17"
+    "-WITH_GFLAGS=OFF"
+  ];
 
   meta = with lib; {
     homepage = "https://github.com/google/glog";