Both work with autotools but they definitely don't mean the same thing, cause
problems with other build systems, and are bad form. Existence should always be
checked with #ifdef or #if defined.
gst_v4l2_clear_error() doesn't work like g_clear_error(), it
doesn't NULLify the pointer, so set freed debug string to NULL
so it doesn't get freed again if gst_v4l2_clear_error() is
called twice on the error.
CID 1362901
This is to allow returning an error that can easily be sent as
message to the application if the element needs it. Using this
also allow ignoring errors.
https://bugzilla.gnome.org/show_bug.cgi?id=766172
