Update CMake to 3.9 (#376)

Co-authored-by: Michael Graeb <[email protected]>

CMakeLists.txt

@@ -1,10 +1,6 @@
-cmake_minimum_required(VERSION 3.1)
+cmake_minimum_required(VERSION 3.9)
 project(aws-c-mqtt C)
 
-if (POLICY CMP0069)
-    cmake_policy(SET CMP0069 NEW) # Enable LTO/IPO if available in the compiler, see AwsCFlags
-endif()
-
 option(ASSERT_LOCK_HELD "Enable ASSERT_SYNCED_DATA_LOCK_HELD for checking thread issue" OFF)
 if (ASSERT_LOCK_HELD)
     add_definitions(-DASSERT_LOCK_HELD)

README.md

@@ -4,13 +4,13 @@ C99 implementation of the MQTT 3.1.1 and MQTT 5 specifications.
 
 ## License
 
-This library is licensed under the Apache 2.0 License. 
+This library is licensed under the Apache 2.0 License.
 
 ## Usage
 
 ### 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.
 
@@ -203,6 +203,6 @@ the wire. For QoS 1, as soon as PUBACK comes back. For QoS 2, PUBCOMP. `topic` a
 ```c
 int aws_mqtt_client_connection_ping(struct aws_mqtt_client_connection *connection);
 ```
-Sends a PINGREQ packet to the server. 
+Sends a PINGREQ packet to the server.
 
 [aws-c-io]: https://github.com/awslabs/aws-c-io