archiveopteryx: fix build on aarch64 by conditioning `-Wno-error` flag

pkgs/servers/mail/archiveopteryx/default.nix

@@ -21,13 +21,14 @@ stdenv.mkDerivation rec {
   '';
 
   # fix build on gcc7+ and gcc11+
-  NIX_CFLAGS_COMPILE = builtins.toString [
+  NIX_CFLAGS_COMPILE = [
     "-std=c++11" # c++17+ has errors
     "-Wno-error=builtin-declaration-mismatch"
     "-Wno-error=deprecated-copy"
     "-Wno-error=implicit-fallthrough"
-    "-Wno-error=mismatched-new-delete"
     "-Wno-error=nonnull"
+  ] ++ lib.optionals (stdenv.cc.isGNU && lib.versionAtLeast stdenv.cc.version "11") [
+    "-Wno-error=mismatched-new-delete"
   ];
 
   buildPhase = ''jam "-j$NIX_BUILD_CORES" '';