Add compatibility with older 2.8 version of CMake

Some legacy systems (Ubuntu 14.04, RHEL7.2) have older versions of CMake.

CMakeLists.txt

@@ -1,13 +1,22 @@
-cmake_minimum_required(VERSION 3.2)
-project(json11 VERSION 1.0.0 LANGUAGES CXX)
+cmake_minimum_required(VERSION 2.8)
+if (CMAKE_VERSION VERSION_LESS "3")
+  project(json11 CXX)
+else()
+  project(json11 VERSION 1.0.0 LANGUAGES CXX)
+endif()
 
 enable_testing()
 
 option(JSON11_BUILD_TESTS "Build unit tests" OFF)
 option(JSON11_ENABLE_DR1467_CANARY "Enable canary test for DR 1467" OFF)
 
-set(CMAKE_CXX_STANDARD 11)
-set(CMAKE_CXX_STANDARD_REQUIRED ON)
+if(CMAKE_VERSION VERSION_LESS "3")
+  add_definitions(-std=c++11)
+else()
+  set(CMAKE_CXX_STANDARD 11)
+  set(CMAKE_CXX_STANDARD_REQUIRED ON)
+endif()
+
 if(CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)
   set(CMAKE_INSTALL_PREFIX /usr)
 endif()