I'm wondering if maybe we should just follow the the Bazel convention here.
They're having 3 attributes (settings):
conlyopts - Add these options to the C compilation command.
copts - Add these options to the C/C++ compilation command.
cxxopts - Add these options to the C++ compilation command.

So copts is already covered by compileOptions, we could just add 2 new settings for the other two.
Searching for custom -std=X setting in LLVM is easy is relatively cheap, we can insert the defaults if there is no custom settings.
However, it would still not fix a big bigger issue - if parts of the (Scala Native C/C++) stdlib require some version od -std, let's say -std=c++14, but users code or it's dependency require newer standard it would still fail.

So maybe we can third alternative - let's define -std setting in the project descriptor so that we can always control our own sources - but allow users to use custom values for everything else. We can play with order or filtering of options coming from project descriptor over these defined by user.

I was thinking of allowing the the project descriptor so the internal libs or third party libs could select their minimum or required version.

I will have to look at the Bezel convention but Lee was making fun of cxx so maybe cpp :-) But why is copt for both C/C++? Maybe I am missing a key point here. I wanted to make it very specific so we can add other things if needed. I can work on naming etc. once I get it going.

I was looking and indeed C++ has other flags specific to C++ so I understand what you were saying about finding the std flag. I will work towards that.