Fix pkg-config files to avoid overlinking

Fix pkg-config files to avoid overlinking

Libraries that are used internally by libFLAC(++) but are not part of
their API should be listed in pkg-config "private" clauses. Otherwise
executables that are linked dynamically against libFLAC(++) will have
unneeded direct dependencies (overlinking).

Based on a patch by Brad Smith from
  https://sourceforge.net/p/flac/bugs/397/
that I updated to only include ogg if libFLAC is actually built with
ogg support.

Signed-off-by: Erik de Castro Lopo <erikd@mega-nerd.com>