simplify top level CMakeLists.txt
diff --git a/cmake/zlib.cmake b/cmake/zlib.cmake
new file mode 100644
index 0000000..16cd9e6
--- /dev/null
+++ b/cmake/zlib.cmake
@@ -0,0 +1,39 @@
+# Copyright 2017 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_ZLIB_PROVIDER}" STREQUAL "module")
+ if(NOT ZLIB_ROOT_DIR)
+ set(ZLIB_ROOT_DIR ${CMAKE_CURRENT_SOURCE_DIR}/third_party/zlib)
+ endif()
+ set(ZLIB_INCLUDE_DIR "${ZLIB_ROOT_DIR}")
+ if(EXISTS "${ZLIB_ROOT_DIR}/CMakeLists.txt")
+ # TODO(jtattermusch): workaround for https://github.com/madler/zlib/issues/218
+ include_directories(${ZLIB_INCLUDE_DIR})
+
+ add_subdirectory(${ZLIB_ROOT_DIR} third_party/zlib)
+ if(TARGET zlibstatic)
+ set(_gRPC_ZLIB_LIBRARIES zlibstatic)
+ endif()
+ else()
+ message(WARNING "gRPC_ZLIB_PROVIDER is \"module\" but ZLIB_ROOT_DIR is wrong")
+ endif()
+ if(gRPC_INSTALL)
+ message(WARNING "gRPC_INSTALL will be forced to FALSE because gRPC_ZLIB_PROVIDER is \"module\"")
+ set(gRPC_INSTALL FALSE)
+ endif()
+elseif("${gRPC_ZLIB_PROVIDER}" STREQUAL "package")
+ find_package(ZLIB REQUIRED)
+ set(_gRPC_ZLIB_LIBRARIES ${ZLIB_LIBRARIES})
+ set(_gRPC_FIND_ZLIB "if(NOT ZLIB_FOUND)\n find_package(ZLIB)\nendif()")
+endif()