transform-dep.py: fake the -MP compiler option

tools/transform-dep.py

@@ -50,6 +50,7 @@ def import_d_file(in_file: str) -> str:
         else:
             out_lines.append(line.replace('\\', '/'))
 
+        headers: List[str] = []
         for line in it:
             suffix = ''
             if line.endswith(' \\\n'):
@@ -59,7 +60,10 @@ def import_d_file(in_file: str) -> str:
                 path = line.strip()
 
             path = convert_path(path)
+            headers.append(path)
             out_lines.append(f"\t{path}{suffix}\n")
+        # the metrowerks compiler doesn't support -MP
+        out_lines.extend([f'{header}:\n' for header in headers])
 
     return ''.join(out_lines)