CONFIG: reinstate FOAM_LD_LIBRARY_PATH adjustments (#2801)

- the special MacOS dlopen handling (commit f584ec97)
  did not fully solve the problem with SIP clearing.

  Eg, sourcing the RunFunctions (for runParallel) triggers SIP and
  clears DYLD_LIBRARY_PATH. With the cleared path it finds the dummy
  libraries: the dummy Pstream::init() fails.