Also, ifnet plugin would read the configuration value, which is just wrong
because:

  - the configuration might not be set and ifnet would fail to fallback
    to the compile time default.
  - the configuration only is in effect if the plugin is also available.
    Otherwise, we fallback to the next plugin.

Only the dhcp-manager knows which DHCP plugin is in use.

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.

Like for the device plugins, check_so_symbols is called from the
Makefile of the plugin itself, not from tests/Makefile.am.

Just keep it consistent.

Don't have the test recompile parts of the settings plugin.
Instead, build one core library that is used both by the test
and the settings plugin.

Advantage: might save some compilation time, but more importantly: the
test use the same object code then NetworkManager itself, avoiding
different behavior due to compilation flags.

First of all, G_LOG_DOMAIN only matters when using g_log() directly.
Inside core, we always want to log via nm-logging. Every call to a
g_log() is a bug in the first place (like a failed assertion that logs
a g_critical() during g_return_if_fail()).

So, for all practic purposes, the logging domain is not used.

For nm-logging, the G_LOG_DOMAIN has no effect. Unless we find a proper
use of this domain, G_LOG_DOMAIN should not differ from what the rest of
core.

At some point, linker on ppc64le would throw away useful symbols along with the
garbage: https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=1c9177d9a5c3e06d3344347c8068acfb7d8ecc8b

Add a post-build check so that the distributors running broken binutils
would catch this early and fix their toolchain (or disable the garbage
collecting at the expense of larger and slower binaries).

(cherry picked from commit 680b3d1037059e57f538c24c1b8b26c15d33906a)