libostree: add versioning macros

OSTree currently provides no way to inspect the versioning
information at run time, being only available at compile
time through pkg-config.

This is a problem for e.g. Flatpak, that needs to check
whether the 'update-frequency' option is available. Checking
at compile time isn't great since it's not looking for new
symbols, but only if an optional feature is present.

This commit, then, adds a new header that is generated
at compile time, exposing OSTree's versioning information.

Closes: #728
Approved by: cgwalters

src/libostree/ostree.h

@@ -33,3 +33,4 @@
 #include <ostree-gpg-verify-result.h>
 
 #include <ostree-autocleanups.h>
+#include <ostree-version.h>