Revert "xz: Use pipe2() if available."

This reverts commit 7a11c4a8e5.
It is a problem when libc has pipe2() but the kernel is too
old to have pipe2() and thus pipe2() fails. In xz it's pointless
to have a fallback for non-functioning pipe2(); it's better to
avoid pipe2() completely.

Thanks to Michael Fox for the bug report.

configure.ac

@@ -650,8 +650,8 @@ gl_GETOPT
 # Find the best function to set timestamps.
 AC_CHECK_FUNCS([futimens futimes futimesat utimes utime], [break])
 
-# These are nice to have but not mandatory.
-AC_CHECK_FUNCS([posix_fadvise pipe2])
+# This is nice to have but not mandatory.
+AC_CHECK_FUNCS([posix_fadvise])
 
 TUKLIB_PROGNAME
 TUKLIB_INTEGER

src/xz/file_io.c

@@ -95,13 +95,7 @@ io_init(void)
 	// we are root.
 	warn_fchown = geteuid() == 0;
 
-	// Create a pipe for the self-pipe trick. If pipe2() is available,
-	// we can avoid the fcntl() calls.
-#	ifdef HAVE_PIPE2
-	if (pipe2(user_abort_pipe, O_NONBLOCK))
-		message_fatal(_("Error creating a pipe: %s"),
-				strerror(errno));
-#	else
+	// Create a pipe for the self-pipe trick.
 	if (pipe(user_abort_pipe))
 		message_fatal(_("Error creating a pipe: %s"),
 				strerror(errno));
@@ -114,7 +108,6 @@ io_init(void)
 			message_fatal(_("Error creating a pipe: %s"),
 					strerror(errno));
 	}
-#	endif
 #endif
 
 #ifdef __DJGPP__