interpreter: allow retrieving build options with get_option()

Fixes: mesonbuild#14788
Signed-off-by: Paolo Bonzini <[email protected]>
(cherry picked from commit bdabc2e)

Author: bonzini

docs/yaml/functions/get_option.yaml

@@ -20,6 +20,11 @@ description: |
   See [`feature` options](Build-options.md#features)
   documentation for more details.
 
+  For options that are [specified
+  per-machine](Builtin-options.md#specifying-options-per-machine)
+  `get_option()` retrieves the value of the option for the
+  build machine if the argument starts with `build.`.
+
 posargs:
   option_name:
     type: str

mesonbuild/interpreter/interpreter.py

@@ -1077,7 +1077,7 @@ def func_get_option(self, node: mparser.BaseNode, args: T.Tuple[str],
         value_object: T.Optional[options.AnyOptionType]
 
         try:
-            optkey = options.OptionKey(optname, self.subproject)
+            optkey = options.OptionKey.from_string(optname).evolve(subproject=self.subproject)
             value_object, value = self.coredata.optstore.get_value_object_and_value_for(optkey)
         except KeyError:
             if self.coredata.optstore.is_base_option(optkey):