Fixed the build failure on Windows. It does not have trunc() defined in...

Fixed the build failure on Windows. It does not have trunc() defined in math.h, so we should not use it code.

sql/field.cc

@@ -5909,6 +5909,7 @@ int Field_str::store(double nr)
   uint length;
   uint local_char_length= field_length / charset()->mbmaxlen;
   double anr= fabs(nr);
+  bool fractional= (anr != floor(anr));
   int neg= (nr < 0.0) ? 1 : 0;
   uint max_length;
   int exp;
@@ -5937,7 +5938,7 @@ int Field_str::store(double nr)
     calculate the maximum number of significant digits if the 'f'-format
     would be used (+1 for decimal point if the number has a fractional part).
   */
-  digits= max(0, (int) max_length - (nr != trunc(nr)));
+  digits= max(0, (int) max_length - fractional);
   /*
     If the exponent is negative, decrease digits by the number of leading zeros
     after the decimal point that do not count as significant digits.