non-conditional use of microbenchmark

[olmjo]

Non-required packages must be used conditionally: see §1.1.3.1 of the manual.

For RcppTN this applies to microbenchmark, which only works on a few platforms.

[olmjo]

https://stackoverflow.com/questions/9899943/is-it-possible-to-use-non-imported-packages-in-a-package-vignette