feat. request: build with gcc/g++

## Abstract

Currently `bc-ur` is builds only with clang, but not gcc/g++ which is the default toolchain on Linux.

This would also allow building `keytool` and `seedtool` with `gcc/g++` which depend on this library.

Start by locating the build configuration that currently requires clang, then inspect how the bc-ur library is used to build keytool and seedtool. Verify the resulting configuration with gcc/g++ and confirm that the library and both tools build successfully.

Written by the indexing model from the issue text.