Seems min C++ standard is now C++17

C++11 no longer builds with the latest ICU

It's a simple fix in the CMakeLists.txt file line 13
```
set(CMAKE_CXX_STANDARD 17)
```

Also adding to the build instructions the fact that ICU_ROOT must be specified e.g.
```
$ cmake .. -DICU_ROOT="C:/Users/.../Downloads/icu4c-75_1-Win64-MSVC2022/"
```
