mcompile: Use the current dir if unspecified

With this running `meson compile` inside the builddir actually works,
and you don't have to do `meson compile -C .`

Clearly this was the intent since the option already has `default='.'`

mesonbuild/mcompile.py

@@ -54,7 +54,6 @@ def add_arguments(parser: 'argparse.ArgumentParser') -> None:
         action='store',
         dest='builddir',
         type=pathlib.Path,
-        required=True,
         default='.',
         help='The directory containing build files to be built.'
     )