Support specifying the "build root" in configure

[travisdowns]

This allows specifying the root of the build directoires, which is currently hardcoded to build. This allows you have several differently-configured builds at once, e.g, build-gcc for gcc, build-clang-14 for clang-14, whatever.

Within the directory, you'll find the usual mode subdirectories, i.e., release, debug, etc.

[travisdowns]

The CI failure looks spurious, probably this one: https://github.com/scylladb/seastar/issues/1127