ngspice.h, cm.h, cleanup NAN defines

src/include/ngspice/cm.h

@@ -45,18 +45,16 @@ NON-STANDARD FEATURES
 #include "ngspice/cmproto.h"
 #include "ngspice/mifcmdat.h"
 
-#if defined (_MSC_VER)
-#ifndef NAN
+#if !defined(NAN)
+#if defined(_MSC_VER) && (_MSC_VER < 1800)
+    /* VS 2012 or less cannot evaluate 0.0/0.0 */
     static const __int64 global_nan = 0x7ff8000000000000i64;
     #define NAN (*(const double *) &global_nan)
+#else
+    #define NAN (0.0/0.0)
 #endif
 #endif
 
-#if !defined(NAN)
-#define NAN (0.0/0.0)
-#endif
-
-
 /*
  * type safe variants of the <ctype.h> functions for char arguments
  */

src/include/ngspice/ngspice.h

@@ -195,11 +195,6 @@ extern double x_atanh(double);
 #define inline __inline
 #define popen _popen
 #define pclose _pclose
-/* NAN not available in MS VS 2008 */
-#ifndef NAN
-    static const __int64 global_nan = 0x7ff8000000000000i64;
-    #define NAN (*(const double *) &global_nan)
-#endif
 
 // undo a #define bool _Bool in MS Visual Studio 2015
 #if defined(bool)
@@ -210,9 +205,14 @@ extern double x_atanh(double);
 #pragma warning(disable: 4127)
 #endif
 
-// for non C99 environments
 #if !defined(NAN)
-#define NAN (0.0/0.0)
+#if defined(_MSC_VER) && (_MSC_VER < 1800)
+    /* VS 2012 or less cannot evaluate 0.0/0.0 */
+    static const __int64 global_nan = 0x7ff8000000000000i64;
+    #define NAN (*(const double *) &global_nan)
+#else
+    #define NAN (0.0/0.0)
+#endif
 #endif
 
 #if defined(__GNUC__)