gtkcssshadowvalue.c: Fix build on older Visual Studio

This code made use of isnan(), which is not provided by pre-2013 Visual
Studio, so fix the build by including fallback-c89.c in place of math.h,
and update fallback-c89.c to include the implementation of isnan() for
Visual Studio, which is copied from the fallback-c89.c in gdk/.

gtk/gtkcssshadowvalue.c

@@ -29,7 +29,7 @@
 #include "gtkrenderprivate.h"
 #include "gtkpango.h"
 
-#include <math.h>
+#include "fallback-c89.c"
 #include <float.h>
 
 #define CORNER_MASK_CACHE_MAX_SIZE 2000U