Force C++ standard to 14

Compilation fails otherwise on macOS in conda-forge with: error: no member named 'auto_ptr' in namespace 'std'

Full log: https://dev.azure.com/conda-forge/84710dde-1620-425b-80d0-4cf5baca359d/_apis/build/builds/812845/logs/51

cpptango is currently compiling with c++14. pytango should use the same.