Log message:
Instead of including bsd.pkg.install.mk directly in a package Makefile,
have it be automatically included by bsd.pkg.mk if USE_PKGINSTALL is set
to "YES".  This enforces the requirement that bsd.pkg.install.mk be
included at the end of a package Makefile.  Idea suggested by Julio M.
Merino Vidal <jmmv at menta.net>.