set rpath for qucsator, qucsconv

* If user provide prefix at configure time, libtool does create the
  rpath into the binary.

* Leaving an empty prefix (default to /usr/local) it does not seem to use rpath.
  Now we use an explicit link command to make sure the rpath is created.
remove message: use rpath, no need to run ldconfig
oxs: add execution permission to postinstall

* it seems to be required on OSX 10.6
Merge branch 'build-fix-rpath'