Add a built-in md5 function

MD5 is widely used in file verification, so
I suggest adding a built-in `md5()` function as
a basic function. For its usage example:

```
md5_str = md5('foobar') # return a md5 string
# some process
```

Author: rruuaanng

docs/yaml/functions/md5.yaml

@@ -0,0 +1,18 @@
+name: md5
+returns: str
+description: |
+  Calculate the 32-bit MD5 value of a given string.
+
+example: |
+  We can call it directly in Meson, and it will return
+  a 32-bit MD5 string constant.
+
+  ```meson
+      md5_str = md5('foobar')
+      message(md5_str) # 3858F62230AC3C915F300C664312C63F
+  ```
+
+posargs:
+  string:
+    type: str
+    description: A string constant or variable.

mesonbuild/ast/interpreter.py

@@ -247,6 +247,7 @@ def __init__(self, source_root: str, subdir: str, subproject: SubProject, subpro
                            'range': self.func_do_nothing,
                            'structured_sources': self.func_do_nothing,
                            'debug': self.func_do_nothing,
+                           'md5': self.func_do_nothing,
                            })
 
     def _unholder_args(self, args: T.Any, kwargs: T.Any) -> T.Tuple[T.Any, T.Any]:

mesonbuild/interpreter/interpreter.py

@@ -404,6 +404,7 @@ def build_func_dict(self) -> None:
                            'unset_variable': self.func_unset_variable,
                            'vcs_tag': self.func_vcs_tag,
                            'warning': self.func_warning,
+                           'md5': self.func_md5,
                            })
         if 'MESON_UNIT_TEST' in os.environ:
             self.funcs.update({'exception': self.func_exception})
@@ -1430,6 +1431,12 @@ def compatibility_sort_helper(s):
         if main_summary:
             main_summary.dump()
 
+    @typed_pos_args('md5', varargs=str, max_varargs=1)
+    @noKwargs
+    def func_md5(self, node: mparser.BaseNode, args: T.Tuple[T.List[str]], kwargs: 'TYPE_kwargs') -> str:
+        from hashlib import md5
+        return md5(args[0][0].encode()).hexdigest()
+
     @noArgsFlattening
     @FeatureNew('warning', '0.44.0')
     @noKwargs