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

apidoc/Makefile.am

@@ -120,6 +120,7 @@ include $(top_srcdir)/gtk-doc.make
 EXTRA_DIST += \
 	version.xml \
 	ostree-sections.txt \
+	ostree-experimental-sections.txt \
 	$(NULL)
 
 -include $(top_srcdir)/git.mk