pyproject.toml (PEP518), build isolation (PEP517) and optional build dependencies

Hi,

I can't use pyproject.toml for some projects with "optional build dependencies" because of build isolation.

However, it would be very useful because we have also real build dependencies.

I created an issue in pip repository (https://github.com/pypa/pip/issues/6144) but it seems to be a more general problem.

I didn't find anything about such potential problem in PEP 517/518. Any thoughts how it can be solved?

In the issue, I propose this:

```
[build-system]
# Minimum requirements for the build system to execute.
requires = [
    "setuptools", "wheel", "numpy", "cython", "jinja2", "fluidpythran>=0.1.7"
]
# packages that are used during the build only if they are installed:
optional = ["mpi4py", "pythran"]

```

Pierre Augier
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