Bump minimum required CMake version

Bump minimum required CMake version to 2.8.12 now that it's released.

That will also avoid the following deprecation warning:

CMake Deprecation Warning at .../civetweb/CMakeLists.txt:2 (cmake_minimum_required):
  Compatibility with CMake < 2.8.12 will be removed from a future version of
  CMake.

CMakeLists.txt

@@ -1,5 +1,5 @@
 # Determines what CMake APIs we can rely on
-cmake_minimum_required (VERSION 2.8.11)
+cmake_minimum_required (VERSION 2.8.12)
 if (${CMAKE_VERSION} VERSION_GREATER 3.2.2)
   cmake_policy(VERSION 3.2.2)
 endif()