We need different export decorators for the different libs.
For now no actual change though, just rename before the release,
and add prelude headers to define the new decorator to GST_EXPORT.
Have to modify libcheck header a bit to avoid warnings
about duplicate 'extern extern'.
Also needs some additions to the libcheck meson.build file
to define CK_EXP_DLL when building the static libcheck.
There's no need to have GstStreamConsistency in a public header for
the time being, so make it private. While we're at it, add a gtk-doc
blurb for it though. Re-fixes #588744.
