ENH: improvements to pythran-config for build system integration

[rgommers]

xref gh-2257. Putting this up for discussion early, with some comments. I considered adding --config compiler.blas=none|openblas support, but that then also has to interact with --cflags etc., so it's not clear that that works well for inclusion in 0.17.1 - and this can be added later.

The --cflags-pythran-only is the only thing we need for SciPy. It allows me to remove all this special handling:
https://github.com/scipy/scipy/blob/b98e79e4b326bf3beb4d861a744101ee7f438471/scipy/meson.build#L103-L138

and replace it with:

pythran_dep = []
if use_pythran
  pythran_dep = dependency('pythran', version: '>=0.17.1')
endif

[serge-sans-paille]

I've removed __PYTHRAN__ in 075cefc

[serge-sans-paille]

I understand that this blas flag is the one you need for scipy, but it should really depend on current config.

You mentioned

I considered adding --config compiler.blas=none|openblas support, but that then also has to interact with --cflags etc., so it's not clear that that works well for inclusion in 0.17.1 - and this can be added later.

In the gommit message but I think that's fine, it should just be also forwaded to extension = pythran.config.make_extension(python=args.python) below

[rgommers]

Okay thanks, I'll see if I can figure out how to introduce this now then.