Replace the deprecated AM_CONFIG_HEADER with AC_CONFIG_HEADERS.

Addresses bug #704 for building on a Mac.  More generically
it addresses the issue building using newers versions of automake.

Tested on OS X 10.8.2 with autotools installed from MacPorts:
automake 1.13.1
autoconf 2.69
glibtoolize 2.4.2

Tested on CentOS 6:
automake 1.11.1
autoconf 2.63
libtoolize 2.2.6b