libsysprof-capture: Move MappedRingBufferSource to libsysprof

As preparation for dropping the GLib dependency from libsysprof-capture,
move the `GSource` which links a `MappedRingBuffer` to a `GMainContext`
from libsysprof-capture to libsysprof.

This requires adding one new piece of API to libsysprof-capture to check
whether the `MappedRingBuffer` is empty.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Helps: #40

src/libsysprof/mapped-ring-buffer-source.h

@@ -0,0 +1,47 @@
+/* mapped-ring-buffer-source.h
+ *
+ * Copyright 2020 Christian Hergert <chergert@redhat.com>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+#pragma once
+
+#if !defined (SYSPROF_INSIDE) && !defined (SYSPROF_COMPILATION)
+# error "Only <sysprof.h> can be included directly."
+#endif
+
+#include <glib.h>
+
+#include "sysprof-version-macros.h"
+
+#include "mapped-ring-buffer.h"
+
+G_BEGIN_DECLS
+
+G_DEFINE_AUTOPTR_CLEANUP_FUNC (MappedRingBuffer, mapped_ring_buffer_unref)
+
+G_GNUC_INTERNAL
+guint             mapped_ring_buffer_create_source      (MappedRingBuffer         *self,
+                                                         MappedRingBufferCallback  callback,
+                                                         gpointer                  user_data);
+G_GNUC_INTERNAL
+guint             mapped_ring_buffer_create_source_full (MappedRingBuffer         *self,
+                                                         MappedRingBufferCallback  callback,
+                                                         gpointer                  user_data,
+                                                         GDestroyNotify            destroy);
+
+G_END_DECLS