Fix detection of _FILE_OFFSET_BITS in CMake

The test wrongly succeeded in C instead of correctly failing in C++
That caused off_t to be different types in core and in modpython on
32-bit systems, and modpython couldn't load with undefined symbol.

cmake/TestWindowsFSeek.cpp

@@ -0,0 +1,12 @@
+// See TestLargeFiles.cmake for the origin of this file
+
+#include <stdio.h>
+    
+int main()
+{
+  __int64 off=0;
+
+  _fseeki64(NULL, off, SEEK_SET);
+        
+  return 0;
+}