build: Add --enable-experimental-api configure option for unstable APIs

There are currently no unstable APIs, but some will be added in
following commits. They will be built and exposed in the libostree
global symbol list iff configured with --enable-experimental-api.

Distributions should not package OSTree with --enable-experimental-api.
This is designed for previewing new APIs on controlled platforms; any of
the APIs hidden behind this option may be changed or removed at any
point.

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

Closes: #832
Approved by: cgwalters

src/libostree/libostree-experimental.sym

@@ -0,0 +1,32 @@
+/*
+ * Copyright © 2017 Endless Mobile, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ *
+ * Authors:
+ *  - Philip Withnall <withnall@endlessm.com>
+ */
+
+/* Symbols in this file are added to the build if OSTree is configured with
+ * --enable-experimental-api. They are not stable or officially supported, and
+ * might disappear or change in future releases. */
+
+/*
+LIBOSTREE_2017.6_EXPERIMENTAL {
+global:
+  some_symbol;
+} LIBOSTREE_2017.6;
+*/