(INCLUDES): Need -I flags for VPATH build.

(clean distclean mostlyclean maintainer-clean): Clean
subdirectories first.
(DISTFILES): Added a bunch of files.
(des_headers): Added desCore rules.
(install-here): Split off target install-headers, which uses $^ to
refer to the files.
(distdir): Use $^ to refer to the files.
distcheck): Fixes.

Rev: src/nettle/Makefile.in:1.2