I hit an assertion failure running with valgrind on a busy machine.
Maybe the timeout is just not long enough for every case.

Increase it.

Keep the include paths clean and separate. We use directories to group source
files together. That makes sense (I guess), but then we should use this
grouping also when including files. Thus require to #include files with their
path relative to "src/".

Also, we build various artifacts from the "src/" tree. Instead of having
individual CFLAGS for each artifact in Makefile.am, the CFLAGS should be
unified. Previously, the CFLAGS for each artifact differ and are inconsistent
in which paths they add to the search path. Fix the inconsistency by just
don't add the paths at all.

Our internal copy of systemd should not be in the search path.
Instead, let users only
  #include "systemd/nm-sd.h"
which then includes everything from systemd that we need.

We want to avoid to accidentally include anything from our
systemd-copy. Any user of that should only include "nm-sd.h",
which then includes everything that is needed further.

For example, "src/devices/wwan/nm-modem-manager.c" has
  #include <systemd/nm-daemon.h>
which in turn includes
  #include "_sd-common.h"
This works all correctly before, because #include "" will first
look in the directory where sd-daemon.h is. However, our mixing of
external systemd library and internal copy is rather dangerous.
Try to avoid it further by keeping the include paths clean.