Makefile: adding files from contrib
* default.mk: Add customizatin variable ORG_ADD_CONTRIB with
descriptiona and commented example.
* targets.mk: Strip leading path and trailing suffix from all elements
of ORG_ADD_CONTRIB, then prefix with contrib/lisp and do wildcard
expansion (this will also remove any invalid names); store the
result in ORG_FROM_CONTRIB. Strip /contrib from ORG_FROM_CONTRIB
and store the result in ORG_TO_LISP. If these are non-empty, copy
the files from contrib/lisp/ into lisp/ before compilation and
remove them when cleanlisp is called.