Merge pull request grpc#21318 from veblush/abseil-build-support

Add abseil targets

bazel/grpc_deps.bzl

@@ -11,6 +11,11 @@ def grpc_deps():
         actual = "@upb//:upb",
     )
 
+    native.bind(
+        name = "absl",
+        actual = "@com_google_absl//absl",
+    )
+
     native.bind(
         name = "absl-base",
         actual = "@com_google_absl//absl/base",

cmake/abseil-cpp.cmake

@@ -0,0 +1,28 @@
+# Copyright 2019 gRPC 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.
+
+if(gRPC_ABSL_PROVIDER STREQUAL "module")
+  if(NOT ABSL_ROOT_DIR)
+    set(ABSL_ROOT_DIR ${CMAKE_CURRENT_SOURCE_DIR}/third_party/abseil-cpp)
+  endif()
+  if(EXISTS "${ABSL_ROOT_DIR}/CMakeLists.txt")
+    add_subdirectory(${ABSL_ROOT_DIR} third_party/abseil-cpp)
+  else()
+    message(WARNING "gRPC_ABSL_PROVIDER is \"module\" but ABSL_ROOT_DIR is wrong")
+  endif()
+elseif(gRPC_ABSL_PROVIDER STREQUAL "package")
+  # Use "CONFIG" as there is no built-in cmake module for absl.
+  find_package(absl REQUIRED CONFIG)
+  set(_gRPC_FIND_ABSL "if(NOT absl_FOUND)\n  find_package(absl CONFIG)\nendif()")
+endif()