Fix libevents standalone build

Fix libevents standalone build
Make sure to build against libtalloc.a and not individual talloc objects,
so that the library can be also built against libtalloc.so and use it as
a shared library instead of statically compiling talloc.
Also add shared-build target to the events library.
Useful to build multiple standalone libraries that depend on each other
without having to install them to the final install dir during the build.