BLD: define _USE_MATH_DEFINES in c_args (#870)

Python.h includes <math.h> before cwt.template.c gets a chance to define
_USE_MATH_DEFINES, so the define never takes effect and M_PI is left
undeclared wherever math.h gates it on that macro.

Author: fjankovi

pywt/_extensions/meson.build

@@ -67,7 +67,7 @@ _cython_tree = [
   fs.copyfile('wavelets_list.pxi'),
 ]
 
-c_args = ['-DPY_EXTENSION']
+c_args = ['-DPY_EXTENSION', '-D_USE_MATH_DEFINES']
 
 libc_wt = static_library('c_wt',
   sources,