%llu format works fine when using from R C API functions like Rprintf, but it fails on Windows when using in plain C like sprintf. We could use PRIu64 instead, defined in inttypes.h.
According to http://r.789695.n4.nabble.com/package-config-file-for-windows-td921218.html inttypes.h is available on CRAN's Windows machines.
According to pocoproject/poco#1426 inttypes.h is available on Solaris.

We need this to be able to construct a message from plain C code, where message body contains length of a vector, thus can be int64. This is used for deferred error/warnings/verbose messages from code that runs in parallel regions.