Have the windows.resource_compiler() preprocesor write a depfile

When using binutils's windres, we can instruct it to invoke the preprocessor
in such a way that it writes a depfile, so that dependencies on #included
files are automatically tracked.

Not implemented for MSVC tools, so skip testing it in that case.

docs/markdown/snippets/windows-resources-dependencies.md

@@ -2,3 +2,6 @@
 
 The `compile_resources()` function of the `windows` module now takes
 the `depend_files:` keyword.
+
+When using binutils's `windres`, dependencies on files `#include`'d by the
+preprocessor are now automatically tracked.

mesonbuild/modules/windows.py

@@ -100,6 +100,11 @@ class WindowsModule(ExtensionModule):
             # Path separators are not allowed in target names
             name = name.replace('/', '_').replace('\\', '_')
 
+            # instruct binutils windres to generate a preprocessor depfile
+            if comp.id != 'msvc':
+                res_kwargs['depfile'] = res_kwargs['output'] + '.d'
+                res_kwargs['command'] += ['--preprocessor-arg=-MD', '--preprocessor-arg=-MQ@OUTPUT@', '--preprocessor-arg=-MF@DEPFILE@']
+
             res_targets.append(build.CustomTarget('Windows resource for ' + name, state.subdir, state.subproject, res_kwargs))
 
         add_target(args)

run_unittests.py

@@ -2397,9 +2397,15 @@ class WindowsTests(BasePlatformTests):
         self.build()
         # Immediately rebuilding should not do anything
         self.assertBuildIsNoop()
-        # Changing mtime of sample.ico should rebuild everything
+        # Changing mtime of sample.ico should rebuild prog
         self.utime(os.path.join(testdir, 'res', 'sample.ico'))
         self.assertRebuiltTarget('prog')
+        # Changing mtime of resource.h should rebuild myres.rc and then prog
+        # (resource compiler depfile generation is not yet implemented for msvc)
+        env = Environment(testdir, self.builddir, get_fake_options(self.prefix), [])
+        if env.detect_c_compiler(False).get_id() != 'msvc':
+            self.utime(os.path.join(testdir, 'inc', 'resource', 'resource.h'))
+            self.assertRebuiltTarget('prog')
 
 
 class LinuxlikeTests(BasePlatformTests):

test cases/windows/5 resources/prog.c

@@ -1,5 +1,7 @@
 #include<windows.h>
 
+// deliberately don't get MY_ICON from resource.h so that depfile generation can
+// be exercised in the WindowsTests.test_rc_depends_files unit test
 #define MY_ICON 1
 
 int APIENTRY