Write the code or build the code? That's always the problem with C++ libraries.

It's not that difficult to write cross platform C++ code. Consuming the dependency is always painful. What build system does it use? What compilers does it support? How do you get the code? There is 0 standardization for these things.

C++ code isn't always that portable either. Old code in a new compiler is very likely to break on warnings, especially if the old code made the high quality decision to hard code -Werror in their build system.

From personal experience Rust as a package and ecosystem is far more portable than C++.