meson: implement building of static programs

meson: implement building of static programs

The whole thing is complicated by the fact that we have two layers
of libraries: e.g. libmount also needs libblkid. If we just tell meson
to make libmount static, this is not enough, because we also need it
to link to a static libblkid. Hence in the case of libs that link to
other libs internally, we need to create a different object with a
a different set of link_with items.

To avoid building the libraries twice, libfdisk and libmount are first
built into an internal "convenience" library, which is then linked into
the static and shared versions as appropriate.