Log Message:
Correctly fix the build breakage on Darwin.

The previous patch removed use of --version-script on Darwin and converted
the shared library extensions to the Darwin standard of .dylib.  The former
is necessary, but the latter was not quite correct.  A distinction must be
made between shared libraries provided by the package for linking of
applications and shared libraries provided by the package for dynamic loading
by Gnome applications.  Gnome applications expect that plugins will use the
.so extension on all Unix platforms, including Darwin.  Thus, a subset of the
shared libraries must be renamed, both on the filesystem and internally.
This is partly mentioned in PR #54824, although that is for graphics/gegl
not tnis package.