gdatetime.c: Fix MSVC builds for lack of NAN items

Use a fallback for isnan() on Visual Studio 2012 or earlier, and define
NAN if it does not exist.

glib/tests/gdatetime.c

@@ -29,6 +29,10 @@
 #ifdef G_OS_WIN32
 #define WIN32_LEAN_AND_MEAN
 #include <windows.h>
+
+#ifndef NAN
+#define NAN HUGE_VAL * 0.0f
+#endif
 #endif
 
 #define ASSERT_DATE(dt,y,m,d) G_STMT_START { \