fix lookup path for tbb stub library on windows (#250)

The stub loader added in #249 resolved tbb.dll via tbbRoot(), which
prefers TBB_LIB when set -- and on Windows, configure always sets
TBB_LIB to the Rtools library directory, which contains only static
TBB libraries. As a result, the lookup silently failed and the stub
was never loaded.

Resolve against the package's own lib directory instead, which is
where install.libs.R places the stub.

Author: kevinushey

R/zzz.R

@@ -20,7 +20,12 @@ loadTbbLibrary <- function(name) {
    # DLL search path.
    if (is_windows()) {
 
-      path <- file.path(tbbRoot(), paste0(name, ".dll"))
+      # NOTE: resolve against the package's own library directory, where
+      # install.libs.R places the stub; tbbRoot() would prefer TBB_LIB,
+      # which on Windows points at Rtools and holds only static libraries
+      arch <- .Platform$r_arch
+      parts <- c("lib", if (nzchar(arch)) arch, paste0(name, ".dll"))
+      path <- system.file(paste(parts, collapse = "/"), package = "RcppParallel")
       if (!file.exists(path))
          return(NULL)