getloadavg, getopt: fix commentary re configure.in

getloadavg, getopt: fix commentary re configure.in

Autoconf is deprecating the name 'configure.in', so change it to
to the new name 'configure.ac' in a couple of places.
* lib/getloadavg.c: configure.in -> configure.ac, in comment.
* m4/getopt.m4 (gl_GETOPT_IFELSE, gl_GETOPT_SUBSTITUTE_HEADER)
(gl_PREREQ_GETOPT): Remove obsolete commentary re Emacs configure.in.
Emacs has renamed it to configure.ac, and it no longer refers
to these macros anyway.