Instead of using multiple recursive Makefile.am files, use a single
Makefile.am that sets and builds all the basic suite of libraries and
binaries for udev. This reduces the number of files in the source tree, and
also reduces drastically the build time when using parallel-make.

With this setup, all the compile steps will be executed in parallel, and
just the linking stage will be (partially) serialised on the libraries
creation.