MAINT: small simplification of meson.build following best practices
Just a couple of things I observed having a quick look at `meson.build`.
@lithomas1  Another cleanup I noticed in the context of the optimization discussion we had is that every extension right now picks up CFLAGS and CPPFLAGS, yet we only have one extension module that actually uses c++ (window/aggregations.pyx)

Would be nice if we didn't mix those up (doesn't need to be done in this PR) thanks @dnicolodi  This broke the windows builds > This broke the windows builds

Didn't CI pass on these changes? How does the build break? The windows builds didn't pass here either I see. I didn't notice the problem before this PR was merged. Sorry for the trouble.

The issue is this:
```
..\..\meson.build:2:0: ERROR: Command `C:\hostedtoolcache\windows\Python\3.9.13\x64\python3.EXE generate_version.py --print` failed with status 1.
```
This is the command that generates the version string in the `project()` call. I changed that from using `python` to `python3`. I don't understand why that fails though. It seems that `python3` is found. If you need a quick resolution, can you please revert just the last commit in this PR, and not the whole thing? Curiously, the `python-dev (windows-latest)` job passes. While the other Windows build fail. I wonder if it is a build environment issue. It would be interesting to see the content of the meson log in `D:\a\pandas\pandas\build\cp311\meson-logs\meson-log.txt` Ah sorry, didn't realize it was the Windows not the npdev that was failing.