[types] Fix check for finite numbers on Linux

On Linux we were merely checking whether f is finite, but that does
not cover the case f=NaN.
=> use isfinite instead which checks whether f != inf && f != NaN

src/types.cpp

@@ -674,11 +674,10 @@ namespace Exiv2 {
     Rational floatToRationalCast(float f)
     {
 #if defined(_MSC_VER) && _MSC_VER < 1800
-        #define isinf(x) (!_finite(x))
-#elif __APPLE__
-        #define isinf(x) (isinf(x) || isnan(x))
+        if (!_finite(f)) {
+#else
+        if (!std::isfinite(f)) {
 #endif
-        if (isinf(f)) {
             return Rational(f > 0 ? 1 : -1, 0);
         }
         // Beware: primitive conversion algorithm