rename octave::build_env namespace from octave::config * build-env-features.sh: Rename from ocgt-conf-features.sh. Update for new names. * build-env.h: Rename from oct-conf.h. Also declare features function. * build-env.in.cc: Rename from oct-conf.in.cc. * oct-conf-features.h: Delete. * libinterp/module.mk: Update. * toplev.cc, __init_gnuplot__.cc, octave.cc: Update for new names.