Rename configure.in to configure.ac, twice

* configure.ac: Rename from configure.in.
* GUILE-VERSION: Update comments; nfc.
* HACKING: Likewise.

* guile-readline/configure.ac: Rename from guile-readline/configure.in.
* guile-readline/LIBGUILEREADLINE-VERSION: Update comments; nfc.

* libguile/__scm.h: Likewise.
* libguile/gen-scmconfig.c: Likewise.
* libguile/gen-scmconfig.h.in: Likewise.

libguile/__scm.h

@@ -32,7 +32,7 @@
  namespace (via adding #define, typedef, or similar to this file) with
  generic names.  This usually means that any new names should be
  prefixed by either SCM_ or GUILE_.  i.e. do *not* #define HAVE_FOO or
- SIZEOF_BAR.  See configure.in, gen-scmconfig.h.in, and
+ SIZEOF_BAR.  See configure.ac, gen-scmconfig.h.in, and
  gen-scmconfig.c for examples of how to properly handle this issue.
  The main documentation is in gen-scmconfig.c.
 

libguile/gen-scmconfig.c

@@ -7,7 +7,7 @@
   Guile has four core headers:
 
     config.h: Guile's private automatically generated configuration
-      header -- generated by configure.in and autoheader.  *NOT*
+      header -- generated by configure.ac and autoheader.  *NOT*
       installed during "make install" and so may not be referred to by
       any public headers.
 
@@ -22,7 +22,7 @@
       the contents of libguile/scmconfig.h.top with the output from
       libguile/gen-scmconfig.  gen-scmconfig bases its output on the
       information in the private config.h header, the contents of
-      gen-scmconfig.h (which is created by configure.in from
+      gen-scmconfig.h (which is created by configure.ac from
       gen-scmconfig.h.in), and the information provided in this file,
       gen-scmconfig.c.
 
@@ -62,9 +62,9 @@
      scmconfig.h.
 
    - in cases where you need to communicate information from
-     configure.in to gen-scmconfig.c, don't add an AC_DEFINE unless
+     configure.ac to gen-scmconfig.c, don't add an AC_DEFINE unless
      you need it for other purposes.  Just add a suitable SCM_I_GSC_*
-     variable to configure.in, set the value, AC_SUBST the value, and
+     variable to configure.ac, set the value, AC_SUBST the value, and
      add an appropriate line to gen-scmconfig.h.in.  All gen-scmconfig
      related AC_SUBST vars should be prefixed with SCM_I_GSC_.
 
@@ -132,7 +132,7 @@ int
 main (int argc, char *argv[])
 {
   pf ("/* This file is automatically generated --"
-      " see configure.in for details */\n"
+      " see configure.ac for details */\n"
       "\n"
       "#ifndef SCM_SCMCONFIG_H\n"
       "#define SCM_SCMCONFIG_H\n");

libguile/gen-scmconfig.h.in

@@ -1,4 +1,4 @@
-/* This file is automatically generated via configure.in. */
+/* This file is automatically generated via configure.ac. */
 
 /* This is the private include header for gen-scmconfig.  See
    gen-scmconfig.c for details.  This file should be considered even