also use _isnan for mingw compiles

git-svn-id: http://svn.osgeo.org/qgis/trunk@10445 c8812cc2-4d05-0410-92ff-de0c093fc19c

Author: mhugent

src/plugins/interpolation/qgsinterpolator.cpp

@@ -23,7 +23,7 @@
 #else
 #include <math.h>
 #endif
-#ifdef _MSC_VER
+#ifdef WIN32
 #include <float.h>
 #define isnan(f) _isnan(f)
 #endif