Rebase to 2.6.4 (#1389575)
    
    This patch rebases flex to upstream version 2.6.4 and thus drops all
    backported upstream patches that the package has been carrying so far.
    
    One exception is flex-2.6.0-yyless.patch which has no equivalent upstream.
    I could not track the origin of this patch because flex development moved
    from SourceForge to GitHub and in the process, sourceforge bug tracker
    content has been lost. However, RHBZ #1281976 seems to suggest that this
    patch was included in order to fix a libreswan build failure caused by a
    flex integer comparison type mismatch bug. I dropped flex-2.6.0-yyless.patch
    and confirmed that the Fedora libreswan package corresponding to
    libreswan-3.25 builds successfully despite this patch being dropped.
    
    Additionally, flex-2.6.4 FTBFS because it uses reallocarray but does not
    define _GNU_SOURCE leading to a missing prototype for reallocarray. This
    commit also includes the upstream patch that fixes this. The patch modifies
    configure.ac, which necessitates executing autoreconf at build time and thus
    a corresponding set of new build dependencies.