Compiler warnings with -Wundef.

[charris]

We are using undefined macros in several places.

[56/359] Compiling C object numpy/_core/_struct_ufunc_tests.cpython-313-x86_64-linux-gnu.so.p/src_umath__struct_ufunc_tests.c.o
../numpy/_core/src/umath/_struct_ufunc_tests.c: In function ‘PyInit__struct_ufunc_tests’:
../numpy/_core/src/umath/_struct_ufunc_tests.c:160:5: warning: ‘Py_GIL_DISABLED’ is not defined, evaluates to ‘0’ [-Wundef]
  160 | #if Py_GIL_DISABLED
      |     ^~~~~~~~~~~~~~~

many places. This is a choice by Python to not define the macro for builds without free threading, but we might want to put a guard on it just to avoid the noise.

../numpy/_core/include/numpy/npy_common.h:504:20: warning: ‘NPY_ULONG’ is not defined, evaluates to ‘0’ [-Wundef]
  504 | #define NPY_UINT64 NPY_ULONG
      |                    ^~~~~~~~~
../numpy/_core/src/multiarray/descriptor.c:2098:22: note: in expansion of macro ‘NPY_UINT64’
 2098 | #if NPY_ULONGLONG == NPY_UINT64
      |                      ^~~~~~~~~~

Probably came about when we dropped the old header. Should be defined in numpyconfig.h

[352/359] Compiling C object numpy/random/_common.cpython-313-x86_64-linux-gnu.so.p/meson-generated_numpy_random__common.pyx.c.o
numpy/random/_common.cpython-313-x86_64-linux-gnu.so.p/numpy/random/_common.pyx.c:2814:35: warning: ‘CYTHON_PROFILE’ is not defined, evaluates to ‘0’ [-Wundef]
 2814 | #if CYTHON_USE_SYS_MONITORING && (CYTHON_PROFILE || CYTHON_TRACE)

Probably a Cython choice not to define the macro.

Also in a test:

limited_api_latest.c:1:23: error: ‘PY_VERSION_HEX’ is not defined, evaluates to ‘0’ [-Werror=undef]
    1 | #if Py_LIMITED_API != PY_VERSION_HEX & 0xffff0000
      |                       ^~~~~~~~~~~~~~
cc1: all warnings being treated as errors

Probably failure to import Python.h