BLD: fix missing build dependency on cython signature .txt files

[rgommers]

Noticed this when looking at gh-18247.

[lysnikolaou]

One question: Are these files used for something else as well? If not, would it be a good idea (or even possible) to remove them from the source tree, regenerate them as part of the build process and add _cython_signature_generator.py as a dependency instead?

[rgommers]

Are these files used for something else as well? If not, would it be a good idea (or even possible) to remove them from the source tree, regenerate them as part of the build process and add _cython_signature_generator.py as a dependency instead?

They're only used during the build process, for the linalg.cython_blas/cython_lapack modules as well as in scipy/_build_utils/_fortran.py for the ILP64 build.

I'd not want to remove them, because to regenerate them requires BLAS/LAPACK sources that aren't available at build time. And vendoring those would be a lot more code than simply committing these two .txt files.

[lysnikolaou]

Oh yes, that's right. Thanks!

[tylerjereddy]

LGTM as well and CI is green