libsForQt5.liblastfm: revert addition of flags on darwin

Apparently on x86_64-darwin there are also C files compiled,
and that doesn't go well with -std=c++*
https://hydra.nixos.org/build/174111870
Fortunately the flag only seems needed with gcc 11 so far.

pkgs/development/libraries/liblastfm/default.nix

···
   buildInputs = [ fftwSinglePrec libsamplerate qtbase ]
     ++ lib.optional stdenv.isDarwin darwin.apple_sdk.frameworks.SystemConfiguration;
 
-  NIX_CFLAGS_COMPILE = [ "-std=c++11" ];
+  NIX_CFLAGS_COMPILE =
+    lib.optionals (stdenv.cc.isGNU && lib.versionAtLeast stdenv.cc.version "11") [
+      "-std=c++11"
+    ];
 
   dontWrapQtApps = true;