Compile Collectd plugin with static link library

* 可參考 iptables plugin 的設定

$ cp libfact.a src/
$ vi src/Makefile.am

if BUILD_PLUGIN_MYPLUGIN
....
myplugin_la_LIBADD = -L. -lfact
....
endif

$ make

References :
Plugin:IPTables - collectd Wiki