meson: Stop using deprecated way getting path of files

Hi,

The just released meson 1.3 strongly deprecated a hack we were using, emitting
a noisy warning (the hack basically depended on an implementation detail to
work). Turns out there has been a better way available for a while, I just
hadn't found it. 1.4 added a more convenient approach, but we can't rely on
that.

Everything continues to work, but the warning is annoying.

The warning:

Message: checking for file conflicts between source and build directory
../home/andres/src/postgresql/meson.build:2972: DEPRECATION: Project uses feature that was always broken, and is now deprecated since '1.3.0': str.format: Value other than strings, integers, bools, options, dictionaries and lists thereof..
[...]
WARNING: Broken features used:
* 1.3.0: {'str.format: Value other than strings, integers, bools, options, dictionaries and lists thereof.'}

I plan to apply this soon, unless I hear some opposition / better ideas / ....

Greetings,

Andres Freund