MSVC builds: Add fallback implementation for pre-2013 MSVC

Pre-2013 MSVC does not have scalbn() and scalbnf(), which are used in the
utility programs.  Add  fallback implementations for these, which can be
used when necessary.

util/options.hh

diff --git a/util/options.hh b/util/options.hh
index 1ef4131..919e4f8 100644
--- a/util/options.hh
+++ b/util/options.hh
@@ -474,5 +474,22 @@
   hb_bool_t show_extents;
 };
 
+/* fallback implementation for scalbn()/scalbnf() for pre-2013 MSVC */
+#if defined (_MSC_VER) && (_MSC_VER < 1800)
+
+#ifndef FLT_RADIX
+#define FLT_RADIX 2
+#endif
+
+__inline long double scalbn (long double x, int exp)
+{
+  return x * (pow ((long double) FLT_RADIX, exp));
+}
+
+__inline float scalbnf (float x, int exp)
+{
+  return x * (pow ((float) FLT_RADIX, exp));
+}
+#endif
 
 #endif