Finally fix the annoying bug which always adds all tags to the

Finally fix the annoying bug which always adds all tags to the
libtool script built by your project configure.  Now we set the
default tags depending on whether AC_PROG_CXX et. al. are actually
called from configure.ac.

* libtool.m4 (_LT_AC_TAGCONFIG): Don't default the tagnames when
no --with-tags argument is given.
(_LT_AC_SHELL_INIT): Factored out from the AC_DIVERT_PUSH code
in...
(_LT_AC_PROG_ECHO_BACKSLASH): ...which now calls the macro
instead.
(AC_LIBTOOL_CXX): Use _LT_AC_SHELL_INIT to add CXX to the
initialisation sequence for tagnames.
(AC_LIBTOOL_GCJ): Similarly for GCJ tag.
(AC_LIBTOOL_RC): Likewise for RC tag.