configure.ac: Support building with old autoconf 2.63

configure.ac: Support building with old autoconf 2.63

This version found on RHEL6 making autoreconf fail with following error:

  configure.ac:61: error: possibly undefined macro: AS_VAR_COPY
  If this token and others are legitimate, please use m4_pattern_allow.
  See the Autoconf documentation.

Apply fix from https://github.com/gdnsd/gdnsd/issues/85 to fix problem.

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: Jozsef Kadlecsik <kadlec@netfilter.org>