Migrate License field to SPDX identifiers
    
    See https://fedoraproject.org/wiki/Changes/SPDX_Licenses_Phase_2
    
    As per guidance, files which are not used by the build and not
    included in the binary RPM were not includes in the new license.
    That excluded a lot of sources in lib/, which are used for
    portability but are not needed for modern glibc-based systems.
    
    Note the m4/ sources are FSFULLR and pulled in via the configure
    process and config.h.in.  While this is very indirect we include the
    m4/ files as "sources" for the purposes of the license, to be
    conservative.