update minimum CMake version to one that does not generate warnings
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6ef9125..74aed2a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,4 +1,4 @@
-CMAKE_MINIMUM_REQUIRED(VERSION 2.8.11)
+CMAKE_MINIMUM_REQUIRED(VERSION 2.8.12)
 CMAKE_POLICY(SET CMP0003 NEW)
 
 PROJECT(picotls)