build: Bump API version automatically each development cycle
Since the API version was added, we've bumped it at some point late-ish in the cycle when enough changes had accumulated (but way after the first ABI break). Automate that process by computing the API version automatically from the project version: With this commit, the new API version will be 5 for the remaining 3.33.x releases and all 3.34.x stable versions; 3.35.1 will then bump it to 6 and so forth.