Log Message:
Fix build breakage on Darwin.

Darwin uses a different extension (.dylib) than other Unixes (.so) for
shared libraries that applications must link against.  However, Gnome
applications expect plugins to use the same extension (.so) on all platforms,
including Darwin.  Consequently, on Darwin some shared libraries must be
renamed, both on the filesystem and internally, and others must use the
correct extension in PLIST.  This is partially mentioned in PR #54824, but
that missed the need for internal renaming of the libraries.  It also
introduced a dependency on the Apple OpenCL framework, which does not seem
to be necessary.

Finally, all the references to Apple-specific OpenCL code must be removed;
previously only some of them were.