src/hb-common.cc: Fix build on older Visual Studio

Visual Studio only supported strtof() from Visual Studio 2013 onwards, so
use strtod() instead to do the operation, which should do the same thing,
sans going to a double, not a float.

src/hb-common.cc

@@ -667,7 +667,11 @@ parse_float (const char **pp, const char *end, float *pv)
   float v;
 
   errno = 0;
+#if defined (_MSC_VER) && (_MSC_VER < 1800)
+  v = strtod (p, &pend);
+#else
   v = strtof (p, &pend);
+#endif
   if (errno || p == pend)
     return false;