version 3.21.3
  - Ensure autogen works in non-srcdir builds
    Only the configure script should be run from the build directory; the
    rest of the ancillary tools that modify the sources and generate the
    configure script itself need to be run inside the source directory.
  - Use AM_DISTCHECK_CONFIGURE_FLAGS
    DISTCHECK_CONFIGURE_FLAGS is for users and packagers to override
    distcheck options, while AM_DISTCHECK_CONFIGURE_FLAGS is for the
    maintainer of the software.

    https://www.gnu.org/software/automake/manual/automake.html#DISTCHECK_005fCONFIGURE_005fFLAGS