Use meson-python to package GSAS-II

[kif]

Suggest a way to compile C, Fortran and Cython files for GSAS-II and install them together with Python files.

There are no C++ files --> why is there a dependency on pybind11 ?
Not all extensions have been converted.
The way the Fortran binding is currently polluting the source tree which is not very nice.
Only linux has been tested so far.

[briantoby]

This is very much of interest. My plan was to move to meson for py3.12, and was waiting for help from someone more experienced with meson. Could you expand on the source tree issue?

[kif]

Well, it is not really an issue but it is not as clean as one would expect:

To compile/bind the fortran code, one needs to call f2py with the proper options which creates 2 additional files, one fortran and the other one C. Then to compile all 3 files together to build the python extension. This is currently done in the source folder, leaving 2 computer generated files in the source folder, moreover untracked in git.

The documentation of f2py (https://numpy.org/doc/stable/f2py/buildtools/meson.html) suggests to use the custom_target() meson function which generates the 2 binding files in the build directory and then the compilation of the 3 files (2 in build directory and one in the source directory) fails. I did not manage to get it working, or maybe I did not try long enough. One option would be to copy the source fortran file to the same place as the binding files and compile them together in the build directory.