Add a new USE_LIBTOOL definition that uses the libtool package instead of
pkglibtool which is now considered outdated.
USE_PKGLIBTOOL is available for backwards compatibility with old packages
but is deprecated for new packages.
XXX One of our pkg-gurus should probably look at this, as
a) /usr/pkg is a the moment hardwired
b) no builtin support for perl/phyton/tcl is yet enabled (I don't want
to build phyton in order to build the pdflib package).
