If you're on the new toolchain with std support already
you'd be unable to build libicu because <cmath> #undefs
some of the defines from <math.h> (e.g. isfinite).
I was trying to port openttd which I ultimately gave up on because
too much of the C++ standard library's functionality is missing at this
point. The libicu library was a dependency for that.
In its current state the libicu port is not thread-safe because of
missing functionality in the C++ standard library (mainly std::mutex,
std::condition_variable, etc.).