Remove MAKE_JOBS_UNSAFE marker: it was added as part of r421281 upon the
port's inception as of version 2.6.2, however, it was neither present in
submitted PR nor explained in the original commit log by pi@. So far my
quick tests do not show evidence that it is parallel-build unsafe.

We'll ask for forgiveness if it is indeed broken against -j X builds and
reinstate MAKE_JOBS_UNSAFE, but with a proper explanation this time.

While here, remove needless += from CMAKE_ARGS and place them at a better
location within the Makefile.

PR: 205377