build: Add --libdir flag to configure

This will allow distribution packages to select an alternative location for the unofficial libnode.so. For example, on Fedora it will install into /usr/lib64 on 64-bit systems.

We've been carrying a similar patch in Fedora for years now, but as of the changes in https://github.com/nodejs/node/pull/42256 it looks like this is now upstreamable.

Signed-off-by: Stephen Gallagher sgallagh@redhat.com