Test code which wants to customize config.h can do so without
touching config.h itself by wrapping config.h in a macro variable
which is passed in to the compiler. It defaults to "config.h" if
no override is provided.
This change would break makefile dependency checking since the selection
of a different header file on the command line is not noticed by make
as a build-trigger. To solve this, we add a layer to the BUILDDIR path
so build products are now specific to the USER_CONFIG choice if it is
not "config.h".
This was a very interesting approach, but for the forseeable
future it's unlikely the code will replace the current one.
However, many parts of it were already moved to the experimental
branch. It turns out the approach with recalculating acceleration
at a constant time interval is exactly right, but works much more
precisely when keeping maths step-based.
Markus Hitter
Phil Hord