environment: allow setting build options with "build." prefix

This is allowed -- it is already deprecated for the cross file, but it should not
assert.  Add a deprecation for the native file too, and remove the assert.

Fixes: d37d649
Fixes: mesonbuild#14789
Signed-off-by: Paolo Bonzini <[email protected]>
(cherry picked from commit 6eb1f2d)

Author: bonzini

mesonbuild/environment.py

@@ -735,6 +735,8 @@ def mfilestr2key(self, machine_file_string: str, section: T.Optional[str], secti
         if section_subproject:
             key = key.evolve(subproject=section_subproject)
         if machine == MachineChoice.BUILD:
+            if key.machine == MachineChoice.BUILD:
+                mlog.deprecation('Setting build machine options in the native file does not need the "build." prefix', once=True)
             return key.evolve(machine=machine)
         return key