Don't rely on dependency in Makefile.inc1 for strfile; make datfiles depend on strfile

In most cases strfile is built as part of build-tools, but in the event that someone
cd'ed to the directory, tried to build from scratch, and had MK_GAMES=no previously,
the build would fail in .../datfiles , trying to find strfile .

Mark this directory tree "SUBDIR_PARALLEL" safe to help facilitate this, instead of
shuffling around the SUBDIR entries (all of the other Makefiles will build standalone).

MFC after: 1 week
Sponsored by: Dell EMC Isilon