Meson build fixes for cross-platform conda-forge packaging

[HaoZeke]

Summary

While enabling Windows and cross-compiled ARM builds on conda-forge, several issues were found in the meson build system that required feedstock-level patches. These should be fixed upstream.

1. with_xtb unnecessarily requires Fortran compiler

File: client/meson.build (line 8)

if get_option('with_fortran') or get_option('with_xtb') or get_option(
    'with_cuh2',
)
    use_fortran = true
    add_languages('fortran', required: true)

with_xtb=True forces add_languages('fortran'), but xtb can be consumed as a pre-built C-API library (e.g. from conda-forge) without a Fortran compiler. Only with_fortran and with_cuh2 should gate Fortran. This blocks Windows MSVC builds where no Fortran compiler is available but xtb is provided as a package.

Fix: Remove get_option('with_xtb') from the Fortran condition.

2. xtb dependency fallback triggers unnecessary source builds

File: client/meson.build (line 329)

xtb_dep = dependency(
    'xtb',
    required: true,
    fallback: ['xtb', 'xtb_dep_static'],
)

When xtb is available as a system/conda package but pkg-config lookup fails (e.g. temporarily), meson falls back to cloning and building xtb from source via subprojects/xtb.wrap. This pulls in the full xtb build tree and requires a Fortran compiler, defeating the purpose of using a pre-built package.

Fix: Remove the fallback parameter, or gate it behind a build option. When xtb is a required system dependency, dependency('xtb', required: true) is sufficient.

3. VLA (variable-length array) in XTBPot.cpp incompatible with MSVC

File: client/potentials/XTBPot/XTBPot.cpp (line 35)

double R_bohr[3 * N];  // N is a runtime parameter

VLAs are a GCC/Clang extension (C99 feature, never standard C++). MSVC rejects this with error C2131: expression did not evaluate to a constant.

Fix: Use std::vector<double> R_bohr(3 * N); and pass R_bohr.data() to C API functions (xtb_newMolecule, xtb_updateMolecule) that expect const double*.

4. Torch library lookup unconditionally requires all libraries

File: client/meson.build (line 433-437)

lib_torch_list = ['c10', 'torch', 'torch_cpu', 'torch_global_deps']
tdeps = []
foreach lib_name : lib_torch_list
    tdeps += cppc.find_library(lib_name, dirs: [LIB_TORCH_LIB_PATH])
endforeach

On Windows, torch_global_deps has no .lib import library (only a .dll). The unconditional find_library fails. The CMake build handles this gracefully by making each lookup optional.

Fix: Use required: false and skip missing libraries:

foreach lib_name : lib_torch_list
    _lib = cppc.find_library(lib_name, dirs: [LIB_TORCH_LIB_PATH], required: false)
    if _lib.found()
        tdeps += _lib
    endif
endforeach

---

All four issues are currently worked around via patches in the conda-forge feedstock (PR #18). Fixing upstream would simplify packaging.