Limit the libraries pulled in when dynamic linking.

See also https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=745543 for a longer explanation. To quote Russ:

The convention for pkgconfig files is to list the libraries required for dynamic linking to get the public ABI in Libs and the other libraries requied for static linking in Libs.private.  This prevents over-linking binaries on systems with good shared library dependency resolution, such as Linux.