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/TestLargeFiles.cpp.cmakein

@@ -0,0 +1,26 @@
+// See TestLargeFiles.cmake for the origin of this file
+
+#cmakedefine _LARGEFILE_SOURCE
+#cmakedefine _LARGEFILE64_SOURCE
+#cmakedefine _LARGE_FILES
+#cmakedefine _FILE_OFFSET_BITS ${_FILE_OFFSET_BITS}
+
+#include <sys/types.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+int main(int argc, char **argv)
+{
+  /* Cause a compile-time error if off_t is smaller than 64 bits,
+   * and make sure we have ftello / fseeko.
+   */
+#define LARGE_OFF_T (((off_t) 1 << 62) - 1 + ((off_t) 1 << 62))
+  int off_t_is_large[ (LARGE_OFF_T % 2147483629 == 721 && LARGE_OFF_T % 2147483647 == 1) ? 1 : -1 ];  
+  FILE *fp = fopen(argv[0],"r");
+  off_t offset = ftello( fp );
+
+  fseeko( fp, offset, SEEK_CUR );
+  fclose(fp);
+  return 0;
+}
+