CMake 3.10 needs C++11

[adridg]

Summary

Release notes at https://blog.kitware.com/cmake-3-10-0-available-for-download/

• Support for building CMake itself with C++98 compilers was dropped. CMake is now implemented using C++11.

Use C++11 then. This has no effect on amd64, x86 where clang was already
the base compiler, with C++11 support, but does affect PowerPC and others.

PR 224721

[tcberner]

As antoine pointed out that the exp-run won't do us any good, just go a head.

[adridg]

This was fixed with r457650