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-writer.c

@@ -97,7 +97,7 @@ struct _SpCaptureWriter
   SpCaptureStat stat;
 };
 
-#ifndef SP_DISABLE_GOBJECT
+#ifdef SP_ENABLE_GOBJECT
 G_DEFINE_BOXED_TYPE (SpCaptureWriter, sp_capture_writer,
                      sp_capture_writer_ref, sp_capture_writer_unref)
 #endif