SCons: List .gen.cpp sources explicitly to avoid globbing errors

Whenever we change the name (or remove) generated cpp files with the `.gen.cpp`
extension, users run into build issues when switching between branches (i.e.
switching before and after the name change/removal). This is because we glob
`*.cpp` so if a now-obsolete file from a previous build is present, we'll
include it too, potentially leading to bugs or compilation failure (due to
missing headers or invalid code).

So globbing patterns in `add_source_files` will now skip files ending with
`.gen.cpp`, which should instead be passed explicitly where they're used.

(cherry picked from commit c133480531)

methods.py

@@ -11,7 +11,7 @@ from SCons.Script import Glob
 from SCons.Variables.BoolVariable import _text2bool
 
 
-def add_source_files(self, sources, files, warn_duplicates=True):
+def add_source_files(self, sources, files):
     # Convert string to list of absolute paths (including expanding wildcard)
     if isbasestring(files):
         # Keep SCons project-absolute path as they are (no wildcard support)
@@ -21,17 +21,20 @@ def add_source_files(self, sources, files, warn_duplicates=True):
                 return
             files = [files]
         else:
+            # Exclude .gen.cpp files from globbing, to avoid including obsolete ones.
+            # They should instead be added manually.
+            skip_gen_cpp = "*" in files
             dir_path = self.Dir(".").abspath
             files = sorted(glob.glob(dir_path + "/" + files))
+            if skip_gen_cpp:
+                files = [f for f in files if not f.endswith(".gen.cpp")]
 
     # Add each path as compiled Object following environment (self) configuration
     for path in files:
         obj = self.Object(path)
         if obj in sources:
-            if warn_duplicates:
-                print('WARNING: Object "{}" already included in environment sources.'.format(obj))
-            else:
-                continue
+            print('WARNING: Object "{}" already included in environment sources.'.format(obj))
+            continue
         sources.append(obj)