pkg-autotools: commonalise the libtool patching code

Currently, the heuristic to patch libtool scripts is duplicated both in
the libtool post-patch hook and the autoreconf pre-configure hook.

Move that code into a single function that gets called from both hooks.

[Thomas: change back the author to Johan. Yann mistakenly changed it
when submitting the patch series.]

Signed-off-by: Johan Oudinet <johan.oudinet@gmail.com>
[yann.morin.1998@free.fr: split the patch into semantically separate
patches]
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>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>