cosmetic: spaces to tabs in m4 macro

To be consistent with the rest of the file and the rest of the macros,
and so identation is displayed properly in a text editor.
build: if Gst# backend is enabled, don't build the unmanaged backend

It could happen that both backends are built, and the user thinks it is
running only the GStreamerSharp backend, when in reality it could be
running both at the same time.