Meson already uses /W2, like it uses -Wall with GCC.

The code generation rules are explicitly built for each supported API
target, but they ought to be refactored since they are pretty much
identical.

In order to do that, we can store the arguments to the custom_target
rules inside an array and then iterate over each element.

This cuts down the complexity of the Meson build, and the chances of
getting something wrong due to duplication.

We depend on an older version of Meson; once we can bump up the minimum
version, we'll be able to fix a couple of less than optimal uses of the
Meson API.

This way, we don't have to depend on Python2 on newer OSes.

Instead of having Meson determine the invocator through the shebang
line we explicitly pass the script file to the Python interpreter.

This will allow us to either use Python3 or Python2, or whatever
Python.exe is available on Windows.

We don't really use it, right now, but it may come in handy later, and
it doesn't cost us anything, since the whole thing is optional anyway.

We don't need Doxygen to care about those.

API added in 1.4 should be annotated as such.