compiler.preprocess should only update the private name per directory

We add a unique ID to each rule we create, to work around the use of
an entire build target with private directory named "preprocess" per use
of the preprocess() method.

But this ID doesn't need to increment every time it is used anywhere --
only when it is used in the same subdir as a previous time. That is the
only case where it could conflict.

By making the increment counter per-subdir, we can avoid potential
frivolous rebuilds when a new preprocess() is added in a different
directory, the build is reconfigured, and all uses in the entire project
tree suddenly get new output paths even if they haven't changed.

mesonbuild/interpreter/compiler.py

@@ -3,6 +3,7 @@
 # Copyright © 2021 Intel Corporation
 from __future__ import annotations
 
+import collections
 import enum
 import functools
 import os
@@ -166,7 +167,7 @@ _COMPILES_KWS: T.List[KwargInfo] = [_NAME_KW, _ARGS_KW, _DEPENDENCIES_KW, _INCLU
 _HEADER_KWS: T.List[KwargInfo] = [REQUIRED_KW.evolve(since='0.50.0', default=False), *_COMMON_KWS]
 
 class CompilerHolder(ObjectHolder['Compiler']):
-    preprocess_uid = itertools.count()
+    preprocess_uid: T.Dict[str, itertools.count] = collections.defaultdict(itertools.count)
 
     def __init__(self, compiler: 'Compiler', interpreter: 'Interpreter'):
         super().__init__(compiler, interpreter)
@@ -773,7 +774,9 @@ class CompilerHolder(ObjectHolder['Compiler']):
         if any(isinstance(s, (build.CustomTarget, build.CustomTargetIndex, build.GeneratedList)) for s in sources):
             FeatureNew.single_use('compiler.preprocess with generated sources', '1.1.0', self.subproject,
                                   location=self.current_node)
-        tg_name = f'preprocessor_{next(self.preprocess_uid)}'
+
+        tg_counter = next(self.preprocess_uid[self.interpreter.subdir])
+        tg_name = f'preprocessor_{tg_counter}'
         tg = build.CompileTarget(
             tg_name,
             self.interpreter.subdir,