capture: avoid GObject in libsysprof-capture-2.a

The point of this file was to be easily usable by applications that want
to generate capture files (or proxy them). The intention here is that we
only have SpCaptureReader/SpCaptureWriter (and associated bits necessary
to support that).

This makes sure that we do not use GObject from the libsysprof-capture-2.a
while still having GType's in the libsysprof-2.so dynamic library.

We also invert the GObject macro check so that the default is to not
register a GType which increases the chances that copy/paste code does
not run into double-registering a GType if used with an application that
links against libsysprof-2. (One such example could be if Gtk uses this
code for timers, as it would break Sysprof itself).

lib/capture/sp-capture-reader.h

@@ -59,7 +59,7 @@ gboolean                           sp_capture_reader_save_as             (SpCapt
                                                                           const gchar         *filename,
                                                                           GError             **error);
 
-#ifndef SP_DISABLE_GOBJECT
+#ifdef SP_ENABLE_GOBJECT
 # define SP_TYPE_CAPTURE_READER (sp_capture_reader_get_type())
   GType sp_capture_reader_get_type (void);
 #endif