pkg-autotools: only apply libtool patch at the right moment

Currently, we apply the libtool patch at post-patch time, always. If we
need to autoreconf, we also apply it after autoreconf is done.

Obviously, in that second case, we do not need to apply it at
post-patch time. This has no impact on the result, as autoreconf will
overwrite the just patched files, and they will be patched yet again.
But it is not nice to the user that will see a double "Patching libtool"
message.

So, we apply the libtool patch at post-patch time only if we are not
autoreconfiguring the package.

[Thomas: fix minor typo s/casse/case/ in the commit log.]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Johan Oudinet <johan.oudinet@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>