libtool: don't clobber host libtool infrastructure

The libtool target package stages its files into the host staging directory
and moves the libltdl library parts from there into the target staging
directory afterwards.

By doing so, the package essentially renders the host libtool infrastructure
unusable, leading to the below error in subsequent package builds:

    libtoolize: $pkgltdldir is not a directory: `.../hostpkg/share/libtool`

Prevent this problem by using a dedicated libltdl install prefix in order to
avoid overwriting and moving away preexisting files belonging to tools/libtool.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>