numbers.c:scm_c_make_polar in guile-2.1.7 breaks on APPLE (macOS 10.12).

Reason: APPLE does not have sincos(), and gcc will optimize sin/cos to use cexp(), and cexp() does not preserve sign of zero.

Patch includes (and attached) for guile-2.1.7.  This uses macOS __sincos().

Example:
In scm_c_make_polar, “gcc -O2”  turns sin(), cos() into cexp(), since cexp(i*x) = cos(x) + i*sin(x):

gcc -O0 =>
LM4339:
	movq	-32(%rbp), %rax
	movd	%rax, %xmm0
	call	_sin
	movd	%xmm0, %rax
	movq	%rax, -8(%rbp)
LM4340:
	movq	-32(%rbp), %rax
	movd	%rax, %xmm0
	call	_cos
	movd	%xmm0, %rax
	movq	%rax, -16(%rbp)

gcc -O2 =>
	pxor	%xmm0, %xmm0
LVL2703:
	call	_cexp

I wrote a little C program to show that cexp() does not preserve the zero-signed-ness:

 cos,sin: +1.000000 -0.000000
__sincos: +1.000000 -0.000000
    cexp: +1.000000 +0.000000

The scm_c_make_polar will use sincos() if available, but macOS does not have sincos(), it has __sincos().

#include <stdio.h>
#include <stdint.h>
#include <math.h>
#include <complex.h>

extern double z, p, n;

int main() {
  double complex c;
  double d, sine, cosine;

  d = z*n;
  printf(" cos,sin: %+f %+f\n", cos(d), sin(d));
  __sincos(d, &sine, &cosine);
  printf("__sincos: %+f %+f\n", cosine, sine);
  c = cexp(CMPLX(0.0, d));
  printf("    cexp: %+f %+f\n", creal(c), cimag(c));
}

double z = 0.0, p = +1.0, n = -1.0;


--- configure.ac.orig	2017-02-18 08:35:06.000000000 -0800
+++ configure.ac	2017-02-18 08:35:26.000000000 -0800
@@ -1152,8 +1152,9 @@
 #   asinh, acosh, atanh, trunc - C99 standard, generally not available on
 #                                older systems
 #   sincos - GLIBC extension
+#   __sincos - APPLE extension
 #
-AC_CHECK_FUNCS(asinh acosh atanh copysign finite sincos trunc)
+AC_CHECK_FUNCS(asinh acosh atanh copysign finite sincos __sincos trunc)
 
 # C99 specifies isinf and isnan as macros.
 # HP-UX provides only macros, no functions.
--- libguile/numbers.c.patch.labs	2017-02-18 08:31:21.000000000 -0800
+++ libguile/numbers.c	2017-02-18 08:34:18.000000000 -0800
@@ -9109,6 +9109,8 @@
      details.  */
 #if (defined HAVE_SINCOS) && (defined __GLIBC__) && (defined _GNU_SOURCE)
   sincos (ang, &s, &c);
+#elif (defined HAVE___SINCOS)
+  __sincos (ang, &s, &c);
 #else
   s = sin (ang);
   c = cos (ang);