update CMakeLists.txt for cmake 3.10

[fschloesser]

I did some improvements to CMakeLists.txt since cmake is on version 3.10 for current ubuntu 18.4

[AntoinePrv]

Thanks, should we simply get rid of the cmake>3.10 code, or is CMake not backward compatible?

[fschloesser]

I would leave it the new code as some of it is useful i guess. When the standard version of cmake has moved on we can delete the compatibility code