This would make builds from source work straightforwardly (including
for testing purposes) and simplify the build instructions for new
contributors -- see changes in build instructions.

Note that there is intentionally no corresponding environment variable
because it is unclear whether e.g. a set but *empty* MPLSYSTEMFREETYPE
environment variable should override a system_freetype = True
in setup.cfg, and because this would make the error message in
checkdep_freetype.c more complex (do we tell the use to unset the
environment variable? or should setting it to "0" or "false" be the same
as unsetting it?).  The design here keeps things simple by having a
single "on" switch.

The config entry is under [libs] to allow for a future system_qhull
entry.