build: check for how to call libtoolize.

build: check for how to call libtoolize.

Previously, we would call libtoolize and if that failed, we would call
glibtoolize since some platforms have the libtoolize program under
that name.

Instead, we check to see if the environment variable LIBTOOLIZE is set
and use that if so. Otherwise, we try to find a program "libtoolize"
on the path and failing that glibtoolize. We alert the user if no
suitable program can be found.