Require c++14 flags in compilation

The [cmake documentation of `CXX_STANDARD_REQUIRED`](https://cmake.org/cmake/help/v3.5/prop_tgt/CXX_STANDARD_REQUIRED.html)
demonstrate that this was previously incorrectly set. This forces the
compiler flag to be specified. In the case of gcc 4.8 it sets
`-std=c++1y`. Newer gcc (since version 5) will sets `-std=c++14`.

buildconfig/CMake/GNUSetup.cmake

@@ -97,7 +97,7 @@ if(WITH_UBSAN)
 endif()
 
 set(CMAKE_CXX_STANDARD 14)
-set(CMAKE_CXX_STANDARD_REQUIRED 11)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 # XCode isn't picking up the standard set above.
 if(CMAKE_GENERATOR STREQUAL Xcode)