Issue#114: check for the presence of isnan and isinf, and provide compat macros on MSCV where _isnan and _finite exist instead.

math_compat.h

@@ -0,0 +1,20 @@
+#ifndef __math_compat_h
+#define __math_compat_h
+
+/* Define isnan and isinf on Windows/MSVC */
+
+#ifndef HAVE_DECL_ISNAN
+# ifdef HAVE_DECL__ISNAN
+#include <float.h>
+#define isnan(x) _isnan(x)
+# endif
+#endif
+
+#ifndef HAVE_DECL_ISINF
+# ifdef HAVE_DECL__FINITE
+#include <float.h>
+#define isinf(x) (!_finite(x))
+# endif
+#endif
+
+#endif