cmake: Fix string substitution index error
```
File "mesonbuild/cmake/interpreter.py", line 293, in postprocess
'Unknown {}_std "{}" -> Ingoring. Try setting the project'
IndexError: Replacement index 2 out of range for positional args tuple
```
@@ -292,8 +292,8 @@ class ConverterTarget:
std = m.group(2)
if std not in self._all_lang_stds(i):
mlog.warning(
- 'Unknown {}_std "{}" -> Ingoring. Try setting the project'
- 'level {}_std if build errors occur.'.format(i, std),
+ 'Unknown {0}_std "{1}" -> Ignoring. Try setting the project-'
+ 'level {0}_std if build errors occur.'.format(i, std),
once=True
)
continue
