Update to 0.12.17

Upstream changes:
Changes in Rcpp version 0.12.17 (2018-05-09)
Changes in Rcpp API:

The random number Generator class no longer inhreits from RNGScope (Kevin in #837 fixing #836).

A spurious parenthesis was removed to please gcc8 (Dirk fixing #841)

The optional Timer class header now undefines FALSE which was seen to have side-effects on some platforms (Romain in #847 fixing #846).

Optional StoragePolicy attributes now also work for string vectors (Romain in #850 fixing #849).

Changes in Rcpp Documentation:

A few old typesetting conventions from the prior Rnw format have been corrected (Peter Hickey in #831; Joris Meys; Dirk)

Two internal links to the introduction published in JSS have been updated to the changed filename given the newer TAS introduction.

Some remaining backticks were replaced with straight quotes (Ralf Stubner in #845).

A citation to the Rcpp introducion in the The American Statistician has been added to the introductory and FAQ vignettes.

Changes in Rcpp version 0.12.16 (2018-03-08)
Changes in Rcpp API:

Rcpp now sets and puts the RNG state upon each entry to an Rcpp function, ensuring that nested invocations of Rcpp functions manage the RNG state as expected (Kevin in #825 addressing #823).

The R::pythag wrapper has been commented out; the underlying function has been gone from R since 2.14.0, and ::hypot() (part of C99) is now used unconditionally for complex numbers (Dirk in #826).

The long long type can now be used on 64-bit Windows (Kevin in #811 and again in #829 addressing #804).

Changes in Rcpp Attributes:

Code generated with cppFunction() now uses .Call() directly (Kirill Mueller in #813 addressing #795).

Changes in Rcpp Documentation:

The Rcpp FAQ vignette is now indexed as 'Rcpp-FAQ'; a stale Gmane reference was removed and entry for getting compilers under Conda was added.

The top-level README.md now has a Support section.

The Rcpp.bib reference file was refreshed to current versions.

(wen)