[CMake] Require C++14

On 16.04, fix:
…/dynamic-graph-python-4.0.0/include/dynamic-graph/python/module.hh:47:26: error: 'exposeEntity' function uses 'auto' type specifier without trailing return type
inline auto exposeEntity() {
^

CMakeLists.txt

@@ -25,6 +25,7 @@ INCLUDE(cmake/python.cmake)
 # Project definition
 COMPUTE_PROJECT_ARGS(PROJECT_ARGS LANGUAGES CXX)
 PROJECT(${PROJECT_NAME} ${PROJECT_ARGS})
+CHECK_MINIMAL_CXX_STANDARD(14 ENFORCE)
 
 # Project dependencies
 FINDPYTHON()