cxops.h and fastrig.h include each other, and depending on the ordering ATAN2
may not be defined here.

--- dttsp-20101018/cxops.h	2010-10-18 21:24:11.000000000 +0100
+++ dttsp-20101018/cxops.h	2025-07-04 19:49:42.913631255 +0100
@@ -142,12 +142,14 @@
   return Cmplx((REAL) (z.re * cos(z.im)), (IMAG) (z.re * sin(z.im)));
 }
 
+#ifdef ATAN2
 PRIVATE INLINE
 COMPLEX
 Cr2p(COMPLEX z) {
   return Cmplx((REAL) hypot(z.re, z.im),
 	       (REAL) ATAN2(z.im, z.re));
 }
+#endif