vla in c++ are rejected by newer clang
it's flagged an extension, and rejected with our current setting.
/host/sieve/las-fbroot-qlattice.hpp:448:23: error: variable length arrays in C++ are a Clang extension [-Werror,-Wvla-cxx-extension]
448 | uint32_t inverses[n_roots];
| ^~~~~~~
Gotta see how to placate clang.