fixes for epel6, epel7, and fedora-rawhide compatibility
    
    * under fedora-rawhide, rpmvercmp is provided by rpmio
    * under epel, we need to compile with -std=gnu99
    * under epel, VCS tag is not supported
    * under epel, we need to use older libgit's object and function names
    * under epel6, libgit contains git_threads_init instead of git_libgit2_init