Update CMake to 3.9 (#489)

Co-authored-by: Michael Graeb <graebm@amazon.com>

Author: waahm7

CMakeLists.txt

@@ -1,10 +1,6 @@
-cmake_minimum_required(VERSION 3.1)
+cmake_minimum_required(VERSION 3.9)
 project(aws-c-http C)
 
-if (POLICY CMP0069)
-    cmake_policy(SET CMP0069 NEW) # Enable LTO/IPO if available in the compiler, see AwsCFlags
-endif()
-
 option(ENABLE_PROXY_INTEGRATION_TESTS "Whether to run the proxy integration tests that rely on pre-configured proxy" OFF)
 option(ENABLE_LOCALHOST_INTEGRATION_TESTS "Whether to run the integration tests that rely on pre-configured localhost" OFF)
 

README.md

@@ -10,7 +10,7 @@ This library is licensed under the Apache 2.0 License.
 
 ### Building
 
-CMake 3.1+ is required to build.
+CMake 3.9+ is required to build.
 
 `<install-path>` must be an absolute path in the following instructions.