fix race condition for creating SAS_DLL_PATH, redo of PR #520

[bmaranville]

A clean re-implementation of the fix in #520 on the current master (underlying function was changed in another commit)

[llimeht]

Note that there's a similar location which also could be racy in sasmodels/core.py:precompile_dlls L362. The same fix is sufficient. It's also unlikely to get hit, but possibly still feasible when making sasmodels documentation with make -j4 for instance.