Failure on cluster._vq build with Meson

[rgommers]

From https://github.com/rgommers/scipy/runs/3778235538?check_suite_focus=true, on macOS there's a failure looking like:

[92/1492] Generating scipy/cluster/_optimal_leaf_ordering with a custom command
[93/1492] Generating scipy/cluster/_vq with a custom command
FAILED: scipy/cluster/_vq.c 
/usr/local/miniconda/envs/scipy-dev/bin/cython -3 --fast-fail ../scipy/cluster/_vq.pyx -o scipy/cluster/_vq.c -I .

Error compiling Cython file:
------------------------------------------------------------
...
"""

cimport cython
import numpy as np
cimport numpy as np
from scipy.linalg.cython_blas cimport dgemm, sgemm
^
------------------------------------------------------------

/Users/runner/work/scipy/scipy/scipy/cluster/_vq.pyx:13:0: 'scipy/linalg/cython_blas.pxd' not found
[94/1492] Compiling C object scipy/cluster/_optimal_leaf_ordering.cpython-39-darwin.so.p/meson-generated_..__optimal_leaf_ordering.c.o
warning: unknown warning option '-Wno-unused-but-set-variable'; did you mean '-Wno-unused-const-variable'? [-Wunknown-warning-option]
1 warning generated.
ninja: build stopped: subcommand failed.

It's likely that there's a dependency issue here; it fails only very rarely but something must be wrong.

[rgommers]

Hmm, another one in https://github.com/rgommers/scipy/pull/75/checks?check_run_id=3781058324

 [209/1492] Compiling C object scipy/special/cython_special.cpython-39-darwin.so.p/sf_error.c.o
warning: unknown warning option '-Wno-unused-but-set-variable'; did you mean '-Wno-unused-const-variable'? [-Wunknown-warning-option]
1 warning generated.
[210/1492] Compiling Cython source scipy/special/_ellip_harm_2.pyx
FAILED: scipy/special/_ellip_harm_2.cpython-39-darwin.so.p/scipy/special/_ellip_harm_2.pyx.c 
cython --fast-fail -3 -o scipy/special/_ellip_harm_2.cpython-39-darwin.so.p/scipy/special/_ellip_harm_2.pyx.c scipy/special/_ellip_harm_2.pyx

Error compiling Cython file:
------------------------------------------------------------
...
from .special cimport cython_special
^
------------------------------------------------------------

scipy/special.pxd:1:0: 'scipy/special/cython_special.pxd' not found

Maybe due to updating to Meson master. Should change that to a fixed commit again, and run latest master only on the nightly build.

[rgommers]

xref gh-31

[rgommers]

This should be related to mesonbuild/meson#8961. Closing as duplicate of gh-31.