Check for existence of sincos

It apparently doesn't exist everywhere.

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

configure.ac

@@ -826,7 +826,7 @@ AC_TYPE_UID_T
 
 # Check for round(), rint(), isnan(), isinf() and nearbyint()
 AC_CHECK_LIB(m,round,,)
-AC_CHECK_FUNCS(round rint nearbyint)
+AC_CHECK_FUNCS(round rint nearbyint sincos)
 AC_CHECK_DECLS([isnan, isinf], [], [], [[#include <math.h>]])
 
 # Checks for gdkspawn