Please add the 'shared' build target #289

Users should be able to choose whether they want static or shared library.

On FreeBSD, for example, there is a policy that static libs should not be installed without their shared counterparts. This is done for manitainability and trackability of dependencies.

The requested change is contained in this patch: https://cgit.freebsd.org/ports/tree/databases/sqlite-ext-vec/files/patch-Makefile

PR: #293