introduce "lib/" in default prefix install for modules
|Target version:||Icinga 1.x - 1.7|
sourcing from #2346, a different install target is needed - being $libdir.
the default prefix does not provide that, but only libexec, which is wrong as event broker modules cannot be run by themselves.
furthermore this will help when we create packages with upcoming icinga-mq, where it should integrate seaminglessly into the $prefix structure.
$ make install /usr/bin/install -c -m 775 -o icinga -g icinga -d /usr/local/icinga/lib
idoutils themselves will also check if lib is created and then install idomod.o over there.