imagemagick: use internal libtool

For some reason, our imagemagick.mk file calls libtool, but assumes
that libtool is available on the host, which may not be
true. Therefore, we use ImageMagick's internal libtool, which has been
used for compiling/linking all the rest of ImageMagic anyway.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>