Made C++17 default.
Made C++17 default.

Cmake version 3.14.0 adds `-std=gnu++11` option to the compiler command line if the `CMakeLists.txt` didn't specify the C++ version. That option overrides `-std=c++1z` options derived from the main `CMakeLists.txt` file. We need to specify the `CXX_STANDARD` option to make c++17 default.
Derecho-Project/derecho-unified,Derecho-Project/derecho-unified,Derecho-Project/derecho-unified,Derecho-Project/derecho-unified,Derecho-Project/derecho-unified