include_next: port to autoconf 2.63

include_next: port to autoconf 2.63

On RHEL 6.4 (using autoconf 2.63), './gnulib-tool --test math'
fails with the following, since commit 74540d44:
executing autoconf
configure:5693: error: possibly undefined macro: AS_VAR_COPY
      If this token and others are legitimate, please use m4_pattern_allow.
      See the Autoconf documentation.

Since we document that we support out-of-the-box use with autoconf
2.59 and later, the fix is to provide a fallback definition.

* m4/gnulib-common.m4 (AS_VAR_COPY): Define if missing.

Signed-off-by: Eric Blake <eblake@redhat.com>