Replace obsolete AC_AIX with AC_USE_SYSTEM_EXTENSIONS

Replace obsolete AC_AIX with AC_USE_SYSTEM_EXTENSIONS

Since Autoconf 2.62 the AC_AIX macro has been made obsolete and should
be replaced with the AC_USE_SYSTEM_EXTENSIONS instead.

Both macro behaviors are the same since the old one is just a wrapper
around the new AC_USE_SYSTEM_EXTENSIONS:
http://git.savannah.gnu.org/cgit/autoconf.git/tree/lib/autoconf/specific.m4

PHP 7.2+ and the main configure.ac script require minimum Autoconf 2.64+.

Refs:
- http://git.savannah.gnu.org/cgit/autoconf.git/tree/NEWS
- https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Obsolete-Macros.html