Let Automake support `configure.ac'.

Let Automake support `configure.ac'.
* automake.in: In the comments, prefer `configure.ac' to
`configure.in'.
When using &am_conf_error, don't repeat `in configure.in', since
the function already reports it.
When referring to `configure.in' use `$configure_ac' instead.
(configure_ac): New global variable.
(scan_configure, scan_one_configure_file): Rename as...
(scan_autoconf_files, scan_one_autoconf_file): this.
(scan_autoconf_files): Compute $configure_ac.
(transform): New function.
(handle_configure): Use it to transform CONFIGURE_AC in
`remake.am' and in `remake-hdr.am'.
(common_files): Also consider `configure.ac'.
Use consistently single quotes.
* aclocal.in (configure_ac): New variable.
Use it.
* texi-vers.am: Depend on CONFIGURE_AC not configure.in.
* remake.am: Likewise.
* remake-hdr.am: Likewise.