when testing, don't build the re2 install path manually

Pluck the RE2 install dir out of the "compileline --libraries" output instead of building it ourselves.  Doing so will automatically adapt to future changes in the install path.

This fixes regexp/ferguson/ctests testing failures due to:
  https://github.com/gbtitus/chapel/commit/e7c4859643c55b5c5dd602249b342cf1d11bba85