Update to NLopt 2.7.1

[astamm]

I am the new maintainer of the nloptr package.
I updated the package to the latest version of NLopt, which is now 2.7.1, and use cmake to build when needed from included sources.
On Windows however, it seems it is not so easy to have cmake as a hard system requirement and, in particular, it is not installed on the winbuilder devel machine.
As a result, I cannot get the new version of nloptr onto CRAN.
Would you accept to update the version on rwinlib so that we can rely on it for building nloptr on windows please?