make sure files arguments to compiler.compiles and friends, performs rebuild

If the compiler check is updated as a string in meson.build, we force
rebuild, which is a good thing since the outcome of that check changes
the configuration context and can enable or disable parts of the build.

If the compiler check came from a files() object then we didn't add a
regen rule on those files.

Fixes #1656

mesonbuild/interpreter/compiler.py

@@ -275,6 +275,7 @@ class CompilerHolder(ObjectHolder['Compiler']):
     def run_method(self, args: T.Tuple['mesonlib.FileOrString'], kwargs: 'CompileKW') -> 'RunResult':
         code = args[0]
         if isinstance(code, mesonlib.File):
+            self.interpreter.add_build_def_file(code)
             code = mesonlib.File.from_absolute_file(
                 code.rel_to_builddir(self.environment.source_dir))
         testname = kwargs['name']
@@ -434,6 +435,7 @@ class CompilerHolder(ObjectHolder['Compiler']):
     def compiles_method(self, args: T.Tuple['mesonlib.FileOrString'], kwargs: 'CompileKW') -> bool:
         code = args[0]
         if isinstance(code, mesonlib.File):
+            self.interpreter.add_build_def_file(code)
             code = mesonlib.File.from_absolute_file(
                 code.rel_to_builddir(self.environment.source_dir))
         testname = kwargs['name']
@@ -457,6 +459,7 @@ class CompilerHolder(ObjectHolder['Compiler']):
         code = args[0]
         compiler = None
         if isinstance(code, mesonlib.File):
+            self.interpreter.add_build_def_file(code)
             code = mesonlib.File.from_absolute_file(
                 code.rel_to_builddir(self.environment.source_dir))
             suffix = code.suffix

unittests/allplatformstests.py

@@ -1965,6 +1965,13 @@ class AllPlatformTests(BasePlatformTests):
         # cxx.links with C source
         self.assertEqual(cmds[3][0], cc)
         self.assertEqual(cmds[4][0], cxx)
+        if self.backend is Backend.ninja:
+            # updating the file to check causes a reconfigure
+            #
+            # only the ninja backend is competent enough to detect reconfigured
+            # no-op builds without build targets
+            self.utime(os.path.join(testdir, 'test.c'))
+            self.assertReconfiguredBuildIsNoop()
 
     def test_ndebug_if_release_disabled(self):
         testdir = os.path.join(self.unit_test_dir, '28 ndebug if-release')