Only add them if --prefix= wasn't specified, and only allow
one path of each to be added.  This way Fink and Darwinports
users won't have libs from *both* (which would conflict).

Testing from OSX (Fink and/or Darwinports) users would be greatly
appreciated, as would testers from other non-GNU/Linux systems.

git-svn-id: https://svn.musicpd.org/mpd/trunk@4432 09075e82-0dd4-0310-85a5-a0d7c8717e4f