BLD: Added interpolate to meson

[czgdp1807]

Reference issue

What does this implement/fix?

Additional information

[czgdp1807]

cc @rgommers

[czgdp1807]

/home/runner/work/scipy/scipy/scipy/interpolate/_ppoly.pyx:14:0: 'scipy/linalg/cython_lapack.pxd' not found

It seems like work is yet to be done to create this extension. I checked, linalg/meson.build. Found the following,

cython_blas = py3.extension_module('cython_blas',
  _generate_cy[0],  # cython_blas.pyx
  # TODO: this mixed source case isn't working yet given Meson Cython support
#  [_generate_cy[0],  # cython_blas.pyx
#   _generate_cy[2],  # cython_blas.pxd
#   _generate_cy[4],  # _blas_subroutines.h
#   'fortran_defs.h',
#  ],
  link_with : fwrappers,
  include_directories : inc_np,
  dependencies : [py3_dep, blas],
  install : true,
  subdir : 'scipy/linalg')

py3.extension_module('cython_lapack',
  _generate_cy[1],  # cython_lapack.pyx
  # TODO: this mixed source case isn't working yet given Meson Cython support
#  [_generate_cy[1],  # cython_lapack.pyx
#   _generate_cy[3],  # cython_lapack.pxd
#   _generate_cy[5],  # _lapack_subroutines.h
#   'fortran_defs.h',
#  ],
  link_with : fwrappers,
  include_directories : inc_np,
  dependencies : [py3_dep, blas],
  install : true,
  subdir : 'scipy/linalg')

[rgommers]

It seems like work is yet to be done to create this extension. I checked, linalg/meson.build. Found the following,

It is created, but adding a dependency on it is difficult. I filed an upstream issue for it, tracked in #31.

I suggest to add a comment to _ppoly that it depends on cython_lapack.pxd and comment out the code to build it. I think it's the very last thing to do to make everything that has the Cython linalg dependency to work; and perhaps a good hack is needed until the issue is addressed in Meson.

[czgdp1807]

The tests are passing.

[rgommers]

LGTM, just pushed a small fix for Fortran build flags. Thanks @czgdp1807!