Define _USE_MATH_DEFINES before including Python.h

[fjankovi]

✳️ Assisted by Claude Opus 5

cwt.template.c:18 defines _USE_MATH_DEFINES immediately before #include "math.h", but <math.h> has already been included by that point, so the define has no effect:

cwt.c:1       #include "cwt.h"
cwt.h:3       #include "common.h"
common.h:23   #include "Python.h"
Python.h:24   #include <math.h>     <-- first include; _USE_MATH_DEFINES not yet defined

cwt.template.c is not a translation unit of its own — it is textually included from cwt.c (lines 9 and 13), after that chain has already run. The include guard is set, so line 19 is a no-op and M_PI at line 65 is undeclared.

Whether that matters is platform-dependent:

• Linux/glibc — unaffected. CPython's autoconf pyconfig.h defines _XOPEN_SOURCE 700, and glibc exposes M_PI under __USE_XOPEN regardless of _USE_MATH_DEFINES.
• Windows — fails. PC/pyconfig.h defines none of _XOPEN_SOURCE, _POSIX_C_SOURCE, _GNU_SOURCE or _USE_MATH_DEFINES, and mingw-w64's math.h gates the M_* block on #if !defined(__STRICT_ANSI__) || defined(_POSIX_C_SOURCE) || ... || defined(_USE_MATH_DEFINES). meson.build sets c_std=c17, which defines __STRICT_ANSI__ — so every clause is false at the first include.

Observed building 1.7.0 from source under Python 3.14 on Windows (that version has no cp314 wheel):

../pywt/_extensions/c/cwt.template.c: In function 'float_pi':
../pywt/_extensions/c/cwt.template.c:65:16: error: 'M_PI' undeclared (first use in this function)
   65 |         return M_PI;

1.9.0 ships cp314 wheels, so the source build is rarely exercised right now — but the code is unchanged on main, and this resurfaces in the window before wheels exist for each new CPython.

This PR moves the define ahead of the first include in cwt.c. The alternative is -D_USE_MATH_DEFINES in _extensions/meson.build's c_args; I chose the source-level define so it also holds for builds that don't go through meson. The existing define in cwt.template.c is left alone — it is now redundant but harmless.

Caveat: I have no Windows toolchain here, so the fix is reasoned from the mingw-w64 header rather than compiled. MSVC is untested. Downstream context: ROCm/TheRock#7798.

[rgommers]

Thanks for the PR @fjankovi! I'd prefer to have the define in meson.build in c_args: - could you please move it there?

It doesn't require a code comment, a one-line fix is enough.