nloptr 2.0.2

This is a patch version in which:

• I link to the nlopt library via nlopt/lib/libnlopt.a instead of
  -Lnlopt/lib -lnlopt when building nlopt from included sources to avoid
  potential mess where -lnlopt could look for the nlopt library in other
  places and possibly link with an existing too old system build of nlopt.

Additionally, we contacted Simon Urbanek for updating the nlopt recipe for
macOS users so that it does now match the latest v2.7.1, which should avoid
nlopt to be built on the fly on CRAN machines.