dependencies/base: Add a new method for setting internal dependencies

When we create a dependency as part of another dependency (say Threads),
we want to pass down most of the methods (like required). Currently
however, there is the possibility that we can pass down invalid keyword
arguments, such as 'method'. This new method is meant to work around
that my simplifying and centralizing how we pass these dependencies
down.

mesonbuild/dependencies/base.py

@@ -25,6 +25,7 @@ import textwrap
 import platform
 import itertools
 import ctypes
+import typing
 from typing import Any, Dict, List, Tuple
 from enum import Enum
 from pathlib import Path, PurePath
@@ -73,6 +74,8 @@ class Dependency:
     @classmethod
     def _process_method_kw(cls, kwargs):
         method = kwargs.get('method', 'auto')
+        if isinstance(method, DependencyMethods):
+            return method
         if method not in [e.value for e in DependencyMethods]:
             raise DependencyException('method {!r} is invalid'.format(method))
         method = DependencyMethods(method)
@@ -176,6 +179,20 @@ class Dependency:
         """
         raise RuntimeError('Unreachable code in partial_dependency called')
 
+    def _add_sub_dependency(self, dep_type: typing.Type['Dependency'], env: Environment,
+                            kwargs: typing.Dict[str, typing.Any], *,
+                            method: DependencyMethods = DependencyMethods.AUTO) -> None:
+        """Add an internal dependency of of the given type.
+
+        This method is intended to simplify cases of adding a dependency on
+        another dependency type (such as threads). This will by default set
+        the method back to auto, but the 'method' keyword argument can be
+        used to overwrite this behavior.
+        """
+        kwargs = kwargs.copy()
+        kwargs['method'] = method
+        self.ext_deps.append(dep_type(env, kwargs))
+
 
 class InternalDependency(Dependency):
     def __init__(self, version, incdirs, compile_args, link_args, libraries, whole_libraries, sources, ext_deps):