(TARGETS, DISTFILES): Added nettle.pdf.

(.texinfo.dvi, .dvi.ps, .ps.pdf): New targets, to build nettle.pdf.
(DOCTARGETS): New variable with targets that shouldn't be deleted
by make clean.
(maintainer-clean-here): New target. Deletes generated
documentation files.

Rev: src/nettle/Makefile.in:1.22