Add missing BUILD_DIR definition to fix build against neu Meson

The debian/rules reads

---> override_dh_auto_configure-indep: debian/stamps/generate_meson_build_system meson configure $(BUILD_DIR) $(confflags) $(indep_flags) <---

With BUILD_DIR undefined, Meson looks at the current directory, which is not a build directory and makes it error out.

Meson did grudginly accept that before but only printed the configuration, so indep_flags were not get actually used.