Bug 670499-gtk/fallback-c89.c: Add fallback for nearbyint()

This adds a C89 implementation for nearbyint() as it is a function that
is only made available in C99.

gtk/fallback-c89.c

@@ -53,4 +53,15 @@ rint (double x)
       return ceil (x - 0.5);
   }
 }
-#endif
+#endif
+
+#ifndef HAVE_NEARBYINT
+/* Workaround for nearbyint() for non-GCC/non-C99 compilers */
+/* This is quite similar to rint() in most respects */
+
+static inline double
+nearbyint (double x)
+{
+  return floor (x + 0.5);
+}
+#endif