Replace obsolete autotool macro

Autoconf v2.69 (the version currently installed by MacPorts) complains
that "AM_CONFIG_HEADER" is obsolete and fails.  The macro it suggests,
AC_CONFIG_HEADERS, works just fine as a drop-in replacement.

Tested on:
 * OS X 10.8.2 (autoconf 2.69)
 * Fedora 16 (autoconf 2.68)