Define _WIN32 for gfortran when building vendored pFUnit on Windows

[djkees]

Summary

scripts/develop.sh's GFE/pFUnit build fails on Windows at the funit-main target: FUnit.F90 guards use pf_RegexFilter on #ifndef _WIN32, but gfortran's -cpp preprocessing does not predefine _WIN32 the way a C/C++ compiler does, so the guard silently takes the wrong branch and tries to use a module that pFUnit's own CMakeLists correctly excluded from the Windows build via CMake's WIN32 variable.

djkees#163

Changes

• scripts/develop.sh: on Windows (detected via uname -s matching MINGW*/MSYS*/CYGWIN*), pass -DCMAKE_Fortran_FLAGS=-D_WIN32 when configuring the vendored GFE build, so FUnit.F90's existing #ifndef _WIN32 guard sees the same platform signal CMake's own WIN32 check does. No change on Linux/macOS.

This depends on nothing else, but djkees#165 currently has a doc change that assumes this fix is in place (it drops the "Windows: currently fails" caveat from CONTRIBUTING.md) — that PR should merge at the same time as or after this one, not before, or CONTRIBUTING.md would briefly overclaim Windows support.

Testing

Reproduced the exact failure from #163, then verified the fix end-to-end from a clean configure:

cmake -S extern/gfe -B build-dev/extern/gfe -G "Unix Makefiles" \
    -DCMAKE_CXX_COMPILER=g++ -DCMAKE_Fortran_COMPILER=gfortran \
    -DCMAKE_INSTALL_PREFIX=build-dev/install -DCMAKE_BUILD_TYPE=Release \
    -DSKIP_MPI=YES -DSKIP_OPENMP=YES -DSKIP_FHAMCREST=YES -DSKIP_ESMF=YES -DSKIP_ROBUST=YES \
    -DCMAKE_Fortran_FLAGS=-D_WIN32
cmake --build build-dev/extern/gfe --target funit-main   # previously failed at FUnit.F90:46 — now builds
cmake --build build-dev/extern/gfe --target install      # completes, produces PFUNITConfig.cmake

Went further and built CEA itself against that install (core-c preset, CEA_BUILD_TESTING=ON) and ran the pFUnit suite on Windows for what's believed to be the first time: ctest -R cea_core_test -V — 124/124 tests passed.

Compatibility / Numerical behavior

• No expected changes to numerical results

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Drafted with Claude's assistance

• The _WIN32-not-predefined-by-gfortran diagnosis was confirmed directly: preprocessing a minimal #ifndef _WIN32 test program with and without an explicit -D_WIN32, using the same gfortran binary from the cea-dev conda env, showed the guard picks the wrong branch without the flag.
• The fix was verified end-to-end, not just configured: a full GFE/pFUnit build+install succeeded where it previously failed with the exact error from #163, and CEA's own cea_core_test pFUnit suite (124 tests) was built and run against that install on Windows, all passing.