Fixed autogen.sh error "configure.ac:6: error: 'AM_CONFIG_HEADER': this macro is obsolete."

Hi

I had the following error while trying to compile on OSX:

```bash
$ ./autogen.sh
+ glibtoolize --copy --force
glibtoolize: putting auxiliary files in `.'.
glibtoolize: copying file `./ltmain.sh'
glibtoolize: putting macros in AC_CONFIG_MACRO_DIR, `m4'.
glibtoolize: copying file `m4/libtool.m4'
glibtoolize: copying file `m4/ltoptions.m4'
glibtoolize: copying file `m4/ltsugar.m4'
glibtoolize: copying file `m4/ltversion.m4'
glibtoolize: copying file `m4/lt~obsolete.m4'
+ aclocal -I m4
configure.ac:6: error: 'AM_CONFIG_HEADER': this macro is obsolete.
    You should use the 'AC_CONFIG_HEADERS' macro instead.
/opt/local/share/aclocal-1.13/obsolete-err.m4:12: AM_CONFIG_HEADER is expanded from...
configure.ac:6: the top level
autom4te: /opt/local/bin/gm4 failed with exit status: 1
aclocal: error: echo failed with exit status: 1
```

So I just replaced AM_CONFIG_HEADER by AC_CONFIG_HEADERS macro in `configure.ac`.

Only tested on OSX 10.8 :blush: 

Hope this helps :smiley: