[patch, libtool] Patch libtool.m4 at top-level for autoconf 2.68

[Tom Tromey]

Joseph> There is no automatic merging at toplevel, only for certain
Joseph> subdirectories (because of a lack of consensus on making GCC the
Joseph> master for all the shared toplevel files).

How about we reopen this and try to get consensus?

I don't remember the last time this came up.  So I don't know what the
arguments are.  However it seems to me that gcc-as-master has worked out
reasonably well for libiberty and other files -- what makes top-level
different?

Joseph> (On which note, the next release of autoconf will warn about use
Joseph> of the obsolete configure.in filename, and the src repository
Joseph> still has a lot of such files that haven't been renamed to
Joseph> configure.ac.)

Just a friendly warning to anybody changing this: src-release refers to
various configure.in files, so it will need an update at the same time.
I suggest grepping the whole tree for "configure.in"...

Tom