In order to allow working with older versions of autoconf we backport
this macro, but only if it's not defined already (by autoconf itself).

This commit can be reverted after we decide support for autoconf 2.61
and below should be deprecated.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>