build: Do not hardcode path to libtool

Some OSes ship with their own libtool and Autotools can use the
LIBTOOL env var to specify the path to libtool. So make it possible to
use this variable as the libtool path for the introspection scanner as
well. This fixes the build on at least OpenBSD.

https://bugzilla.gnome.org/show_bug.cgi?id=726570