hb-private.hh: Add fallback implementation for round()

For pre-C99 compilers that do not support round(), we need to have a
simplistic implementation for it, when it is not detected during build
configuration, either via CMake or autotools, by using floor() and
ceil(), which are provided in the pre-C99 compilers.

Please see discussion at commit 86a0ac28 for more details for re-adding
this patch.

src/hb-private.hh

@@ -1070,4 +1070,17 @@ struct hb_string_t
 };
 
 
+/* fallback for round() */
+#if !defined (HAVE_ROUND) && !defined (HAVE_DECL_ROUND)
+static inline double
+round (double x)
+{
+  if (x >= 0)
+    return floor (x + 0.5);
+  else
+    return ceil (x - 0.5);
+}
+#endif
+
+
 #endif /* HB_PRIVATE_HH */