- build.{sh,subr } cleanup including Cygwin compatibility fixes and [gs]et_var_dyn().
    - Replaced --build-{scripts,steps} w/ -r, --tarball w/ -t, and --help w/ -h.
    - Create BUILD_{FINISHED,STARTED}_AT_..., BUILD_IN_PROGRESS, and TARBALL_STARTED_AT
      files and BUILD_FINISHED_AT symlink in $PREFIX whenever appropriate.