BLD: define _USE_MATH_DEFINES in c_args

[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.

Fixed by adding -D_USE_MATH_DEFINES to c_args in pywt/_extensions/meson.build.

Caveat: this was not compiled on Windows — the mechanism is derived from the mingw-w64 header rather than observed under MSVC, which 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.

[fjankovi]

✳️ Assisted by Claude Opus 5

Moved to c_args and dropped the code comment — the PR is now a single one-line change to pywt/_extensions/meson.build.

Also rebased onto current main (c55a14e); the branch had been cut before c2c6f23 and 4026e04 landed.