- aclocal.m4: we don't keep auto-generated files (says Wayne)
- pull in required gnulib macros to avoid the overhead of using gnulib's build process and
  requiring gnulib
- socklen is now an own m4 module (socklen.m4)
- HAVE_SOCKLEN_T is now gl_TYPE_SOCKLEN_T
- add havelib module (lib-ld.m4, lib-link.m4, lib-prefix.m4)

tested with:
- aclocal (GNU automake) 1.12.4
- autoconf (GNU Autoconf) 2.69