Fix CMake minimum required version

The "C++17" value of the `CXX_STANDARD` target property, which was
introduced in bitcoin-core#25, is available in
CMake 3.8 and newer.

Author: hebasto

CMakeLists.txt

@@ -2,7 +2,7 @@
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
-cmake_minimum_required(VERSION 3.0)
+cmake_minimum_required(VERSION 3.8)
 project("Libmultiprocess" CXX)
 include(CMakePushCheckState)
 include(CheckCXXSourceCompiles)