message: Fix sprintf build on my machine.
  isnanf/isinff doesn't exist for me, and afaict they're not standard C
  functions.
  I get a warning about `if (av)` when `av` is a float.  Made explicit.
  GPC: sprintf-build
diff: @@ -439,9 +439,9 @@ static int FormatLong(FieldInfo *info, long long val, char *str, int max) {
  static int FormatFloat(FieldInfo *info, float val, char *str, int max) {
  // Check for outliers
  const char *bad = 0;
  - if (isnanf(val))
  + if (isnan(val))
  bad = (val < 0) ? "-nan" : "nan";
  - if (isinff(val))
  + if (isinf(val))
  bad = (val < 0) ? "-inf" : "inf";
  if (bad) {
  for (int i = 0; i < max && bad[i]; i++)
  @@ -542,7 +542,7 @@ static int FormatExp(FieldInfo *info, float val, char *str, int max) {
  int exp = 0;
  float av = fabsf(val);
  - if (av) {
  + if (av != 0) {
  exp = (int)floorf(log10f(av));
  val *= powf(10.0f, static_cast<float>(-exp));
  }
  