The SHARED option will pass the --shared argument to the configure
script, which will cause the build to produce both the node binary and
the libnode.so shared library. Take cues from other packagers (Fedora,
for example) and patch tools/install.py to install both. The
tools/install.py script installs libnode.so with a version suffix equal
to the NODE_MODULE_VERSION defined in src/node_version.h, so the port will
create the libnode.so.## -> libnode.so symlink in the post-install target.

This feature has been requested a couple of times, and it makes sense
to enable it by default.