[10176] Use std::isnan in the hope more non-MS platforms provide it.

Note that G3D::isNaN() is simply wrong IMHO...and not even inlined.
One day, there will be a usable C++ standard not light-years behind good old C...

src/shared/vmap/BIH.h

@@ -34,7 +34,9 @@
 #define MAX_STACK_SIZE 64
 
 #ifdef _MSC_VER
-	#define isnan(x) _isnan(x)
+    #define isnan(x) _isnan(x)
+#else
+    #define isnan(x) std::isnan(x)
 #endif
 
 using G3D::Vector3;