Define FILE_CNT() macro on OpenBSD (FILE is opaque)

Index: src/h/config-internal.h.in
--- src/h/config-internal.h.in.orig
+++ src/h/config-internal.h.in
@@ -178,6 +178,9 @@
 #endif
 
 #undef FILE_CNT
+#if defined(__OpenBSD__)
+#  define FILE_CNT(fp)  (__freadahead((fp)))
+#endif
 #if @ECL_FILE_CNT@ == 1
 #  define FILE_CNT(fp)  ((fp)->_IO_read_end - (fp)->_IO_read_ptr)
 #endif
