Check for existence of sincos

It apparently doesn't exist everywhere.

https://bugzilla.gnome.org/show_bug.cgi?id=745578

tests/gtkgears.c

@@ -36,6 +36,8 @@
  */
 
 #define _GNU_SOURCE
+#include "config.h"
+
 #include <math.h>
 #include <stdlib.h>
 #include <string.h>
@@ -47,6 +49,15 @@
 #define VERTICES_PER_TOOTH 34
 #define GEAR_VERTEX_STRIDE 6
 
+#ifndef HAVE_SINCOS
+static void
+sincos (double x, double *_sin, double *_cos)
+{
+  *_sin = sin (x);
+  *_cos = cos (x);
+}
+#endif
+
 /**
  * Struct describing the vertices in triangle strip
  */