meson: Stop using deprecated way getting path of files

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.

Reviewed-by: Tristan Partin <tristan@neon.tech>
Discussion: https://postgr.es/m/20231129185053.s6c7f73eg7b4ztfi@awork3.anarazel.de
Backpatch: 16-, where the meson build was added.

meson.build

@@ -2925,8 +2925,12 @@ potentially_conflicting_files = []
 foreach t : potentially_conflicting_files_t
   potentially_conflicting_files += t.full_path()
 endforeach
-foreach t : configure_files
-  t = '@0@'.format(t)
+foreach t1 : configure_files
+  if meson.version().version_compare('>=0.59')
+    t = fs.parent(t1) / fs.name(t1)
+  else
+    t = '@0@'.format(t1)
+  endif
   potentially_conflicting_files += meson.current_build_dir() / t
 endforeach
 foreach sub, fnames : generated_sources_ac

src/backend/utils/init/miscinit.c

@@ -107,9 +107,8 @@ InitPostmasterChild(void)
 
 	/*
 	 * Set reference point for stack-depth checking.  This might seem
-	 * redundant in !EXEC_BACKEND builds; but it's not because the postmaster
-	 * launches its children from signal handlers, so we might be running on
-	 * an alternative stack.
+	 * redundant in !EXEC_BACKEND builds, but it's better to keep the depth
+	 * logic the same with and without that build option.
 	 */
 	(void) set_stack_base();