Allow building CHIP without lwip checked out

This used to work prior to project-chip#2325, which added an unconditional
dependency on lwip even if we're using sockets. Make it conditional.

src/BUILD.gn

@@ -15,6 +15,7 @@
 import("//build_overrides/chip.gni")
 
 import("${chip_root}/gn/chip/tests.gni")
+import("${chip_root}/src/lwip/lwip.gni")
 
 config("includes") {
   include_dirs = [
@@ -46,11 +47,12 @@ if (chip_build_tests) {
       "${chip_root}/src/transport/tests",
     ]
 
+    if (chip_with_lwip) {
+      deps += [ "${chip_root}/src/lwip/tests" ]
+    }
+
     if (current_os != "zephyr") {
-      deps += [
-        "${chip_root}/src/lib/shell/tests",
-        "${chip_root}/src/lwip/tests",
-      ]
+      deps += [ "${chip_root}/src/lib/shell/tests" ]
     }
   }
 }

src/inet/BUILD.gn

@@ -19,6 +19,7 @@ import("//build_overrides/nlio.gni")
 
 import("${chip_root}/gn/chip/buildconfig_header.gni")
 import("${chip_root}/gn/chip/tests.gni")
+import("${chip_root}/src/lwip/lwip.gni")
 import("${chip_root}/src/platform/device.gni")
 import("inet.gni")
 
@@ -97,11 +98,14 @@ static_library("inet") {
   public_deps = [
     ":inet_config_header",
     "${chip_root}/src/lib/support",
-    "${chip_root}/src/lwip",
     "${chip_root}/src/system",
     "${nlio_root}:nlio",
   ]
 
+  if (chip_with_lwip) {
+    public_deps += [ "${chip_root}/src/lwip" ]
+  }
+
   if (chip_inet_config_enable_raw_endpoint) {
     sources += [
       "RawEndPoint.cpp",

src/lwip/BUILD.gn

@@ -16,8 +16,11 @@ import("//build_overrides/chip.gni")
 import("//build_overrides/lwip.gni")
 
 import("${chip_root}/gn/chip/buildconfig_header.gni")
+import("${chip_root}/src/lwip/lwip.gni")
 import("${lwip_root}/lwip.gni")
 
+assert(chip_with_lwip)
+
 declare_args() {
   # lwIP platform: standalone, freertos.
   lwip_platform = ""

src/lwip/lwip.gni

@@ -0,0 +1,18 @@
+# Copyright (c) 2020 Project CHIP Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+declare_args() {
+  # Have the the lwIP library available.
+  chip_with_lwip = current_os != "zephyr"
+}

src/lwip/tests/BUILD.gn

@@ -16,8 +16,9 @@ import("//build_overrides/chip.gni")
 
 import("${chip_root}/gn/chip/chip_test.gni")
 import("${chip_root}/gn/chip/chip_test_group.gni")
+import("${chip_root}/src/lwip/lwip.gni")
 
-assert(current_os != "zephyr")
+assert(chip_with_lwip)
 
 chip_test("TestLwIP") {
   sources = [ "tests.c" ]

src/system/system.gni

@@ -12,9 +12,13 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import("//build_overrides/chip.gni")
+
+import("${chip_root}/src/lwip/lwip.gni")
+
 declare_args() {
   # Use the lwIP library.
-  chip_system_config_use_lwip = current_os == "freertos"
+  chip_system_config_use_lwip = chip_with_lwip && current_os == "freertos"
 
   # Use BSD/POSIX socket API.
   chip_system_config_use_sockets = current_os != "freertos"