Bumped nettle so-versions to 3.0. Set hogweed

so-versions to 1.0. New makefile conditionals IF_SHARED and
IF_HOGWEED. Renamed WITH_PUBLIC_KEY to WITH_HOGWEED. Deleted
SHLIBTARGET, SHLIBINSTALL, RSA_EXAMPLES and RSA_TOOLS.

Rev: nettle/configure.ac:1.2