ninja backend: Fix usage of same constants file for native and cross

For example:
```
meson builddir \
        --native-file vs2019-paths.txt \
        --native-file vs2019-win-x64.txt \
        --cross-file vs2019-paths.txt \
        --cross-file vs2019-win-arm64.txt
```
This was causing the error:

> ERROR: Multiple producers for Ninja target "/path/to/vs2019-paths.txt". Please rename your targets.

Fix it by using a set() when generating the list of regen files, and
add a test for it too.

1 files changed, 7 insertions, 7 deletions

diff --git a/mesonbuild/backend/backends.py b/mesonbuild/backend/backends.py
index e43c8c7..eb4d03f 100644
--- a/mesonbuild/backend/backends.py
+++ b/mesonbuild/backend/backends.py
@@ -1191,14 +1191,14 @@ class Backend:
     def get_regen_filelist(self) -> T.List[str]:
         '''List of all files whose alteration means that the build
         definition needs to be regenerated.'''
-        deps = [str(Path(self.build_to_src) / df)
-                for df in self.interpreter.get_build_def_files()]
+        deps = OrderedSet([str(Path(self.build_to_src) / df)
+                for df in self.interpreter.get_build_def_files()])
         if self.environment.is_cross_build():
-            deps.extend(self.environment.coredata.cross_files)
-        deps.extend(self.environment.coredata.config_files)
-        deps.append('meson-private/coredata.dat')
+            deps.update(self.environment.coredata.cross_files)
+        deps.update(self.environment.coredata.config_files)
+        deps.add('meson-private/coredata.dat')
         self.check_clock_skew(deps)
-        return deps
+        return list(deps)
 
     def generate_regen_info(self) -> None:
         deps = self.get_regen_filelist()
@@ -1210,7 +1210,7 @@ class Backend:
         with open(filename, 'wb') as f:
             pickle.dump(regeninfo, f)
 
-    def check_clock_skew(self, file_list: T.List[str]) -> None:
+    def check_clock_skew(self, file_list: T.Iterable[str]) -> None:
         # If a file that leads to reconfiguration has a time
         # stamp in the future, it will trigger an eternal reconfigure
         # loop.