vcl: fix Android build

See <https://code.google.com/p/android/issues/detail?id=74835>,
Android's <cmath> currently does not provide std::copysign(), add a
forwarding stub till it gets fixed.

Change-Id: I49cc74ad1b61fed0460588150f21471d63e9069f

vcl/source/filter/sgvspln.cxx

@@ -23,6 +23,17 @@
 #include <sgvspln.hxx>
 #include <cmath>
 
+#if defined(ANDROID)
+namespace std
+{
+template<typename T>
+T copysign(T x, T y)
+{
+    return copysign(x, y);
+}
+}
+#endif
+
 extern "C" {
 
 /*.pn 277 */