Build: Use non-legacy setuptools backend

Use the modern `setuptools.build_meta` backend rather than the legacy
backend that is implied by the lack of `build-backend`.

This is documented in PEP 517:

> If the `pyproject.toml` file is absent, or the `build-backend` key is
> missing, the source tree is not using this specification, and tools
> should revert to the legacy behaviour of running `setup.py` (either
> directly, or by implicitly invoking
> the `setuptools.build_meta:__legacy__` backend).

https://peps.python.org/pep-0517/#source-trees

Author: mgorny

pyproject.toml

@@ -1,5 +1,6 @@
 [build-system]
 requires = ["setuptools", "wheel"]
+build-backend = "setuptools.build_meta"
 
 [project]
 name = "pygccxml"