Rename option --with-aix-soname to --enable-aix-soname.

Rename option --with-aix-soname to --enable-aix-soname.

Reported at <https://savannah.gnu.org/support/index.php?110674>.

* doc/libtool.texi (LT_INIT): Talk about --enable-aix-soname instead of --with-aix-soname.
* m4/ltoptions.m4 (_LT_WITH_AIX_SONAME): Use AC_ARG_ENABLE instead of
AC_ARG_WITH. In the default case, use _AC_ENABLE_IF, in order to still
recognize the old option --with-aix-soname.
* m4/libtool.m4 (_LT_SYS_DYNAMIC_LINKER): Update comment.
* configure.ac: Update comment.
* NEWS: Mention the change.