This will be used by clang-tidy.

Should be provided by the engine.

This is not perfect yet; we can't use these for linking the library
.so's themselves (there is some duplication) and some of the more
annoying engine dependencies (-ldl) are still scattered across the
Makefiles. Also, maybe we should include the other -Wl flags here.

But we don't want to reimplement pkg-config here, this already gets us
where we need to be right now.

Needed to install pEpIPsecCTypes on a completly clean $PREFIX, before
any dependencies.

This leads to local headers being preferred to installed ones, which is
important when we update headers.

The big new thing are functions to recursively search headers/sources.

The magic option is -x objective-c++.

We could just run every test binary and see if it returns 0. But that
would not work for the pEpRPC tests, which are kinda broken and also the
majority of test binaries so far...

The engine makefiles are notoriously bad at installing everything that
other projects need, so we just allow poking right in there.

We might also fix install bugs in out buildscripts, but this is also
very useful for developers.

This is really a bit like GNUstep make now, but much simpler.
Other projects will need to locate this makefile somehow, either using
an environment variable ($MAKEFILE_COMMON) or by relative path to this
repo (next to the other repos).