configure script under ${WRKSRC}. The symlink is called "libtool.m4",
which is the name expected by the autoconf tools. The symlinking is
only done if ${AUTOCONF_REQD} == 2.13 and USE_LIBTOOL is defined. This
allows autoconf-2.13, autoreconf-2.13 and aclocal-2.13 to use the older
libtool.m4 file when regenerating files. This is okay, because we later
override the generated libtool file anyway.
This fixes problems when libtool-1.5.x is installed and a package tries
to invoke autoconf-2.13 programs at pre-configure time, e.g., gnome-games,
xpuyopuyo, faad2, libwww, etc. These packages no longer need to be
worked around by bumping AUTOCONF_REQD to 2.50.
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