Fix build with automake 1.13

This change fixes the build with automake-1.13. Without this change, the build fails with the following error:

Running aclocal-1.13...
configure.ac:13: error: 'AM_CONFIG_HEADER': this macro is obsolete.
    You should use the 'AC_CONFIG_HEADERS' macro instead.
/usr/share/aclocal-1.13/obsolete-err.m4:12: AM_CONFIG_HEADER is expanded from...
configure.ac:13: the top level
autom4te: /usr/bin/m4 failed with exit status: 1
aclocal-1.13: error: echo failed with exit status: 1