Patch utfcpp build

[bigerl]

As promised to @mike-lischke , here is the rebase of #3040 :

It is unnecessary that each time the external dependency utfcpp is pulled, its tests are built and run. Also its samples are unnecessarily generated.
I suggest to use the provided cmake to disable this behavior.
You can find the documentation of the cmake options here: https://github.com/nemtrif/utfcpp/blob/v3.1.1/CMakeLists.txt#L10-L12

Let's make this build a little bit more green and not use unnecessary resources.

[mike-lischke]

Thanks @bigerl.

@parrt This is a small C++ build change and ready to be merged.