AC_LIBTOOL_DLOPEN (libtool 1.4.3) broken with autoconf 2.56

[R.L. Horn]

It looks as though new versions of autoconf aren't defining AC_PROVIDE_*
macros, upon which AC_LIBTOOL_DLOPEN and AC_LIBTOOL_WIN32_DLL depend.

The fix would appear to be to replace occurences of:
  ifdef([AC_PROVIDE_whatever], ...

where they occur in libtool.m4 (there's only three of them) with:
  AC_PROVIDE_IFELSE([whatever], ...

At least this seems to have things working for me.

Of course, this will probably blow up older autoconfs...