FindPNG: Honor old PNG_LIBRARY if provided (#14398)

In commit 2a797539 (FindPNG: improve library detection, 2013-07-27) we
split the search for PNG into separate PNG_LIBRARY_DEBUG and
PNG_LIBRARY_RELEASE variables.  However, if a project or user sets the
old PNG_LIBRARY value we must honor it instead of searching.

While at it, mark PNG_LIBRARY_RELEASE and PNG_LIBRARY_DEBUG as advanced
and remove a stray debug message.
Merge topic 'FindPNG-compatibility'

6816044 FindPNG: Honor old PNG_LIBRARY if provided (#14398)