BUG: create the build directory before trying to write files in it

Do not try to write the cross file in the build directory before
ensuring that the build directory exists. Add a test to make sure
this does not break again.

Fixes #406.

Author: dnicolodi

mesonpy/__init__.py

@@ -656,6 +656,10 @@ def __init__(
             raise ConfigError(f'Could not find ninja version {_NINJA_REQUIRED_VERSION} or newer.')
         self._env.setdefault('NINJA', self._ninja)
 
+        # make sure the build dir exists
+        self._build_dir.mkdir(exist_ok=True, parents=True)
+        self._install_dir.mkdir(exist_ok=True, parents=True)
+
         # setuptools-like ARCHFLAGS environment variable support
         if sysconfig.get_platform().startswith('macosx-'):
             archflags = self._env.get('ARCHFLAGS', '').strip()
@@ -696,10 +700,6 @@ def __init__(
             for key, value in meson_args.items():
                 self._meson_args[key].extend(value)
 
-        # make sure the build dir exists
-        self._build_dir.mkdir(exist_ok=True, parents=True)
-        self._install_dir.mkdir(exist_ok=True, parents=True)
-
         # write the native file
         native_file_data = textwrap.dedent(f'''
             [binaries]