When we link static libraries together, there must be no duplicate
symbols.

Since we have a lot of static/intermediate libraries, getting this right
is complicated and sometimes leads to ugly solutions.

As a new rule: don't let static libraries link with other static
libraries. Only binaries and libnm/libnm.la should explicitly link
with all the static libraries that they require.

There are exceptions: "src/libNetworkManager.la" and "libnm/liblibnm.la".
These are static, internal libraries, but they are basically *everything*
that ends up in "src/NetworkManager" and "libnm/libnm.la", respecitively.
Hence, these static libraries also link against other static libraries.
Another exception to this rule is "src/libNetworkManagerTest.la", for
similar reasons.