Hack optimal_bufsize for posix platform so that I returns values which
cannot overflow int.

diff -ur gawk-3.1.1.orig/posix/gawkmisc.c gawk-3.1.1.pok/posix/gawkmisc.c
--- gawk-3.1.1.orig/posix/gawkmisc.c	Tue Apr 16 13:58:41 2002
+++ gawk-3.1.1/posix/gawkmisc.c	Wed May 22 11:14:36 2002
@@ -87,7 +87,7 @@
 	if (S_ISREG(stb->st_mode)
 	    && 0 < stb->st_size && stb->st_size < DEFBLKSIZE) /* small file */
 		return stb->st_size;
-	return DEFBLKSIZE;
+	return (DEFBLKSIZE > 24576 ? 24576 : DEFBLKSIZE);
 }
 
 /* ispath --- return true if path has directory components */