refactor Makefile logic for speculative makes of re2/gmp

[trivial, not reviewed]

* pushed logic relating to the third-party dir into the
third-party Makefile.

* made the re2 rules in third-party/Makefile more
  symmetric with gmp and others in order to improve
  the 'up-to-date' message

* moved the speculative builds after the compiler
  build and before the runtime to be more similar
  to when the required third-party packages are
  built. 

I'd hoped that by doing this move, I'd only get the
speculative build message when the build was actually
not already done (by putting the install directory in
as a precondition), but I'd forgotten that that would
change the order in which things were built.  Future
work could do this by moving the test into the actions,
but that felt too big to add tonight.

In addition, found that the chpl_gmp.py script needed the
moral equivalent of Kyle's commit in #431 -- apparently
I didn't test GMP as closely as RE2 the first time around.