automake AM_PROG_AR

Fixed `autogen.sh` error
````
automake: warnings are treated as errors
/usr/share/automake-1.13/am/ltlibrary.am: warning: 'multiload.la': linking libtool libraries using a non-POSIX
/usr/share/automake-1.13/am/ltlibrary.am: archiver requires 'AM_PROG_AR' in 'configure.ac'
lxpanel/Makefile.am:12:   while processing Libtool library 'multiload.la'
...
```
I also [packaged](https://aur.archlinux.org/packages/xfce4-multiload-nandhp-plugin-git/) it for archlinux. I'll be happy switch back to you git if you fix the build