fix missing extension in command path

On Windows, if the native file contains a path without the
extension, the executable may be found, but custom target would
fail because that path does not exist.

mesonbuild/programs.py

@@ -55,6 +55,15 @@ def __init__(self, name: str, command: T.Optional[T.List[str]] = None,
                 if ret:
                     self.command = ret + args
                 else:
+                    if os.path.isabs(cmd) and not os.path.exists(cmd):
+                        # Maybe the name is an absolute path to a native Windows
+                        # executable, but without the extension. This is technically wrong,
+                        # but many people do it because it works in the MinGW shell.
+                        for ext in self.windows_exts:
+                            trial_ext = f'{cmd}.{ext}'
+                            if os.path.exists(trial_ext):
+                                cmd = trial_ext
+                                break
                     self.command = [cmd] + args
         else:
             if search_dirs is None: