core: Add runtime ostree_check_version()

[Previously](https://github.com/ostreedev/ostree/pull/728) we added compile-time
checking for versions, but there are use cases for runtime checking as well,
because in a number of API calls we use `GVariant` as an API extension
mechanism.

Closes: #735
Approved by: jlebon

src/libostree/libostree.sym

@@ -386,12 +386,10 @@ global:
  *                         NOTE NOTE NOTE
  */
 
-/* Stub section for new version, uncomment when the first symbol is added
-LIBOSTREE_2017.$NEWVERSION {
+LIBOSTREE_2017.4 {
 global:
-	someostree_symbol_deleteme;
-} LIBOSTREE_2017.2;
-*/
+  ostree_check_version;
+} LIBOSTREE_2017.3;
 
 /* Stub section for the stable release *after* this development one; don't
  * edit this other than to update the last number.  This is just a copy/paste