I want the same feature in CMake: variable_watch

I transfer from CMake to Meson and accept that Meson has no user defined functions and uses only global variables.

However, I cannot find the similar CMake `variable_watch` function in Meson. I have to spread `warning` function calls in the meson.build files and try to find when and where a variable is used or changed. It is boring and valueless work.

I think it is not hard to add the similar CMake `variable_watch` function in Meson. It is just to add some logic to read and write operations of the global variable table.

variable_watch
https://cmake.org/cmake/help/latest/command/variable_watch.html

Read the linked CMake variable_watch documentation first, then inspect Meson’s global variable table and the warning calls currently spread through meson.build files. Determine the intended behavior for variable reads and writes, including how a watch is registered and what output it should produce. Done means the behavior is defined and verified for the relevant global-variable operations.

Written by the indexing model from the issue text.