autoconf: Prevent autoreconf man page from being rebuilt

The inreplace on autoreconf.in changes the timestamp, causing the man
page to be rebuilt (which would require help2man).  Run inreplace on the
man page as well to prevent that.  It also keeps the man page consistent
with the program.

Closes #24222.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>