libpcap: allow building with RDMA protocol support

disabled by default for a couple reasons:
1. enabling would cause a mass-rebuild.
2. libpcap is an indirect input to rdma-core, hence a circular
   dependency. but rdma-core doesn't actually use libpcap: this loop
   will be removed by <https://github.com/NixOS/nixpkgs/pull/498294>.

pkgs/by-name/li/libpcap/package.nix

@@ -10,7 +10,9 @@
   libnl,
   libxcrypt,
   pkg-config,
+  rdma-core,
   withBluez ? false,
+  withRdma ? false,
   withRemote ? false,
 
   # for passthru.tests
@@ -46,6 +48,7 @@ stdenv.mkDerivation (finalAttrs: {
     bash
   ]
   ++ lib.optionals stdenv.hostPlatform.isLinux [ libnl ]
+  ++ lib.optionals withRdma [ rdma-core ]
   ++ lib.optionals withRemote [ libxcrypt ]
   ++ lib.optionals withBluez [ bluez ];
 
@@ -63,6 +66,9 @@ stdenv.mkDerivation (finalAttrs: {
   ++ lib.optionals stdenv.hostPlatform.isDarwin [
     "--disable-universal"
   ]
+  ++ lib.optionals withRdma [
+    "--enable-rdma"
+  ]
   ++ lib.optionals withRemote [
     "--enable-remote"
   ]