In a future commit, we will split out the build instructions for our
library directory and move them into a subdirectory. One of the benefits
is fixing scoping issues, where e.g. defines do not leak to build
targets where they do not belong to. But unfortunately, this does also
pose the problem of how to propagate some defines which are required by
both the library and the test suite.

One way would be to create another variable keeping track of all added
defines and declare it inside of the parent scope. While this is the
most obvious and simplest way of going ahead, it is kind of unfortunate.
The main reason to not use this is that these defines become implicit
dependencies between the build targets. By simply observing a define
inside of the CMakeLists.txt file, one cannot reason whether this define
is only required by the current target or whether it is required by
different targets, as well.

Another approach would be to use an internal header file keeping track
of all defines shared between targets. While configuring the library, we
will set various variables and let CMake configure the file, adding or
removing defines based on what has been configured. Like this, one can
easily keep track of the current environment by simply inspecting the
header file. Furthermore, these dependencies are becoming clear inside
the CMakeLists.txt, as instead of simply adding a define, we now call
e.g. `SET(GIT_THREADSAFE 1)`.

Having this header file though requires us to make sure it is always
included before any "#ifdef"-preprocessor checks are executed. As we
have already refactored code to always include the "common.h" header
file before any statement inside of a file, this becomes easy: just make
sure "common.h" includes the new "features.h" header file first.