accelerate versionsCompatibilityTest

accelerate versionsCompatibilityTest

by allowing parallel build of units,
and reducing optimization levels.

Parallel build is only effective on "recent" versions of `zstd`,
as previously, the list of units was passed as a list of source files,
which is something neither `make` nor `gcc` can parallelize.
So its impact is mildly effective (-20%).

Reducing optimization level to `-O1` makes compilation much faster.
It also makes runtime slower,
but in this test, compilation time dominates run time.
The savings are very significant (-50%).

On my test system, it reduces the length of this test from 13mn to 5mn.